I think I'd try the cheap ways before I pulled a head - an new performance high flow 180 degree thermostat and a new performance 13-15 pound radiator cap. Get quality performance parts
, not Auto-Zone or similar companies that sells foreign made junk.
If it still does the Old Faithful act, then go for a good compression test - including a leak down test which should tell you conditions in each cylinder - head gasket (and rings/valves). Some of the clone Chevy engines that Oldsmobile and Pontiac
used did have some head cracking problems at the middle cylinders.