Engine Cooling problem
2000 Pontiac Montana 6 cyl Front Wheel Drive Automatic 125000 miles

van has normal tempature when idling, when you start to drive it races up to the the over heating stage on the guage and will slow the van down. water pump recently replaced. also had headgasket replaced. shop that did both repairs says they pressure tested it and it checks out alright. serpentine belt new to. still think there's a compression leak somewhere. when the shop did the headgasket had to weld the head because it was pretty pitted. help!!!