Friend of mine is having the same issue with his Galaxy,he's done stat and pressure cap and like yours it'll do hundreds of miles (towing tin tent mostly) and then he'll do a Shortish trip solo and it'll play up,he's done chemical test on coolant for exhaust gases but that was ok.next step is head off but if gasket is sound then pressure test of head will follow
I haven't changed the stat or pressure cap as I tested the stat and that appears to be good and there is generally a big release of pressure when I unscrew the pressure cap.
What I did do today was discover a silly little plastic plug with a hole in it which lives in the pipe which connects to the top of the expansion bottle. After much prodding and poking I managed to extract it from it's hiding place and find that the hole was blocked. Cleaned it, put it back together, started the engine and the coolant (and gas) started flowing. After much burping and farting I had to add another litre of coolant and the interior heater started working
Marvelling at another piece of design failure from VW
and puzzling over whether the crap in the tiny orifice was
- the original cause of the overheating
- where the crap came from
- whether it came from a failing head gasket
- whether the overheating has caused any permanent damage
Either way I don't have the confidence to take the family on any extended journeys in it in the near future so we went out looking for a replacement / back up mummy bus