Before you start worrying about exhaust gasses in the coolant check out:
1)  Is the system bled properly?  Air in the system the most common cause
2)  Thermostat (already mentioned)
3)  The radiator!!!  I fought cooling problems for years on my '85 Westy
with all the 'experts' (NOT on this list, local VW shops) jumping to the
conclusion it was leaking head gaskets:  "These things are famous for that
problem".  It was a partially clogged radiator, and eventually cost me an
engine.
The reason I bring this up is I have rarely seen people suggest radiator
problems as the cause of overheating and coolant overflow.