AudiForums.com - can anyone help !!

hi can anyone figure this out !! my car is a 2002 a4 1.9tdi avf engine. drive car and after either 5min-30 min when going steady in 5th about 2500 revs, power loss like no turbo car can still rev up to 4000rev and will go to 80mph but no turbo power until I switch off ignition and restart, has done this 5 times and still no dash lights to warn me no codes stored even when I plug scanner in at side of road while engine running in this limp mode as such,I can still get car to rev up to 4000 but takes long time without turbo. I have had turbo off cleaned and ring is working fine, also renewed pipe work and no air leaks only two problems I see are the brake lights were staying on and switch was loose and refitted so they don't stay on all the time and work as they should also the temp gauge goes from under half to zero to 50 and so on. also oil level on stick is on the min. I was wondering because the car wont put any lights on the dash or store any faults could it be because of a faulty brake pedal switch or faulty temp gauge? also I was told on some cars the low oil can make car shut off turbo? have driven car with maf unplugged and is worse straight away with it unplugged I am at a loss cant understand why it wont report a fault code when it does this as I have made it do it 5 times at least and cant get it to put code out, I am not sure if it is limp mode as I can get car to rev up to 4000 and drive at 80 mph but it takes ages?? any help would be great also it seems to do it more when either going up or down a hill