I'm with MPS, sounds like crash damage rusting.
I have had my golf since new and in 100k miles (05 car) the only non service issues have been driveshaft boot, aircon pump and DV, so I am pretty pleased, in my book 100k miles with just the above is a pretty reliable car, I have no issues with build quality, better than my previous car an M3 which cost over double! Has the car been well looked after?If yes can't see why is has been unreliable. I have had five golfs in last ten years, mk5 GT TDI, my current mkV GTI, a MkV Ed 30 (went with my ex :-( ) and two mk IV GTI TDI's all have been v v reliable and all very high build quality, maybe I have just been lucky! Market residuals say not, high residuals = high quality sought after car.