I cop some silly stuff with my MK4 from time to time as well and this is what I always do first...
Remove the battery leads and give them a good clean...leave them off for about 20 mins or so. Flip the lid on the black plastic box that sits on the top of the battery and check that all the connections are clean and tight...any problems in there will be very obvious.
Make sure there is no moisture in that box and give it a good dose of WD40 before closing it again.
Then I trace back any and all leads from that top box and make sure they are connecting properly to whatever they component or earth they are supposed to. You might be surprised at what you find!
Re connect the battery, start up and go for a drive...keep your fingers crossed. May not solve your problem, but it won't cost you anything to try.