Intermitant power drop and low MPG

[thai-wronghorse]

I'm new to derv so could do with some advice on what to check out or replace...
The car is infact a 1999 mk3 TDI 90bhp wagon but thought I'd ask in here seeing as the mk3 section doesn't have a specific TDI section.

Basically the engine will run and pulls well throughout the gears but very occasionally power will drop off and feels really flat even with the throttle pushed to the floor regardless if i even drop down a gear.
This will generally last for 5-10mins and be fine again which I find rather odd and might not do it for another couple of days.
I've also noticed the fuel consumption is going up in the couple of weeks I've owned the car with the average MPG on the first fill up been 47 and the second fill up been 43, I know these engines are good for 55+ so something obviously isn't quite right here?

i'm going to stick the code reader on it tomorrow but wondered if there was any common issues to look out for?

[thai-wronghorse]

It would appear that the thermostat may be goosed as it never really goes much higher than 70 degrees unless I drive in traffic for ages then it'll peak around 90 but drop as soon as hit open road again.
This might explain the drop in MPG but that much, im not convinced. Then theres the power loss still to diagnose...

I stuck the code reader on this afternoon and it threw up a few codes:

00553 - mass air flow sensor G70
00575 - manifold absolute pressure sensor
00626 - glow plug indicator light 29
65535 - control unit faulty / power failure to control unit/ no faults recorded (if MIL not illuminated)

These were cleared as these may have been on the ECU long before I picked the car up and took it for a 20 minute drive at various speeds and rechecked the codes again. All I have now is 65535 which im pretty unsure about plus whilst i was out driving I had none of the power dropping symptoms?