Seriously weird problem with GTI 16V

[dazmatic]

Basically, I've been having some seriously weird problems with my 16 valver.

I had previously asked over at VWVortex but I'm getting nowhere there so what I will do is brief over everything.

Car started hesitating ~2k RPM, even when cold and just pulling out from the road where I live.
Tested TPS and not a single dead spot on it, works a treat.
Then went on to change the CPS and still no change.
Swapped out the coil for another, still no change
Throttle body also cleaned

The problem seems to have changed however from a hesitation at ~2k RPM, to no hesitation whilst driving at all, but once temp reaches high enough for the fan to turn on, the engine cuts out intermittently, as in, it'll cut out but just before it dies completely recovers itself and will eventually cut out completely.

If I then leave it for a few seconds, it'll start and run fine until of course the temp gets high again.

So, since then I have also changed ECU relay and fuel pump relay for known working pulls from another car.
Then changed the fuel filter and no change and today changed the fuel pump and no change.

However, I noticed something particularly strange.
Whilst having my VAGCOM on the car and using the measuring blocks and the car running, as usual it got up to temperature and started intermittently cutting out like usual, but when it did I lost communication with the VAGCOM, as if the ECU was switching on and off sporadically.
Now, the ECU switching on and off would explain the cutting out but what supplies the ECU with it's power?

NOW FOR THE WEIRD BIT
Obviously, first thought was the ECU relay failing (again?) so I decided to pull the ECU relay and replace the contacts with a pair of spade terminals soldered via cable to a switch to simulate the ECU relay contacts opening and closing.

When running the car on the drive, I flicked the switch off and the car continued to run! Eventually it got up to 'temperature' and started cutting out and recovering like usual so and I could sit there and flick this switch all I like simulating the ECU relay contacts and it made no difference.

It's like the ECU is parasitically getting power from elsewhere.

Now, I am aware that in a previous life with a previous owner that it did have an alarm/immobiliser fitted...

Oh my god, could it be the immobiliser failing?!

[murraymint]

My car was doin this, changed the coil, still doing it, new ht leads sorted it.

[dazmatic]

Oh yeah, leads were replaced maybe 10k miles ago for new, as was the coil, plugs etc.

[murraymint]

Ah right. Can't help you then sorry, might be worth pm'ing DanChave as he has had a lot of running problems with his and he might be able to help

[dazmatic]

There must be a reason as to why my car can run without the ECU relay in it?

How does the immobilizer work?
I've got a Laserline immobilizer but I'm not sure whether that would cut the ECU out entirely or just prevent the car from starting?

[Screech16v]

defo pulling number 30 relay?

[dazmatic]

Definitely relay 30! I have 2 of them, original one from the car and the 2nd hand pull and have both in my hand!

[dazmatic]

Ok,

Now it's getting EVEN weirder... just went out to the car.
The car will now start without even putting the immobiliser keyfob into the slot as in, the immobiliser doesn't even work now.


EDIT:
Just been outside to the car again as when I was looking at the relay box the other day, it was a complete birds nest behind it!
I had noticed the other day that there was a black cable with an inline fuse just handing there, so, thinking that it was for the old scorpion alarm system which is no more, snipped the loose cable off.
There was also a few other snipped cables behind there, which, look snipped I.E. no exposed ends.
There was one however, a black cable, that did have an exposed end and it didn't occur to me this morning thinking about this immobiliser malarkey that maybe perhaps it had something to do with it.

Thinking that car started even without the immobiliser and no flashy red light on the fob front, lead me to think that the immobiliser wasn't actually working.

So, I went out there with a multimeter and tested between the black cable I snipped and this black cable that had the exposed end. Turns out there was 12V between them, so I figured theres a fuse, whats the harm in connecting them.

So I rewire this fuse end to the stripped cable that comes out from the back of the fuse box and the other end of the fuse to the cable I snipped. I gingerly, placed the fuse in the holder and screwed the 2 together successfully with no pops or bangs.

I look up, turn the key to start and nothing! So the immobiliser is working again, but, the car will still start without relay 30 BUT only after placing my keyfob in the holder.

I'm thinking that perhaps the black cable somehow worked loose from the inline fuse turning off the immobiliser and that perhaps, parasitically, the ecu was drawing power from the immobiliser hence why it will still run without relay 30 in.

So, I will put relay 30 back in and take it for a spin and see if I break down!

[Screech16v]

So now it starts does it cut out still when up to temp?

[dazmatic]

Ok...

The immobiliser now immobilises, but, the original hesitation problem is back AND now when up to temperature it cut out completely, left me stranded on the side of the road again.

So, im going to scrap it next week!