intermittent coolant leak - 323F GLX V6

Hi

Ive got a 323F GLX V6 '95 model with an intermittent coolant leak.

It will most often occur after Ive had a long continuous drive and then slow
for traffic.  I see steam billowing out from the bonnet and theres a strong
smell like rotton eggs,

I called out the RAC on one occasion and the guy thoroughly checked the
cooling system for leaks. He pressure checked it, checked the fan etc and
couldnt find anything wrong.  I never get any signs of overheating.

Ive tried RadWeld and keep a check on the coolant level.

It definetly seems to be related to when the engine is very hot. Is there
some sort of pressure relief valve that opens?

Any ideas?

Thanks for your time.

Ian