An S-15 4.3L I just acquired maint. responsibility for is overheating at idle or on the road. Symptoms:

1. Goes to about 260 on the gauge, "Check gauges" light comes on.
2. Some coolant movement through radiator when run with cap off. Not a bubbling movement as with my '95 Ford 4.0L, but movement - wouldn't call it good flow.
3. Temp gauge barely gets off bottom if run without thermostat, radiator coolant movement about the same without cap, but gets hot and smells of overheating.
4. Heater blows hot. Even get some hot air in cab without heater switch on at upper temp gauge levels.
5. Water pump moving coolant, not sure volume is sufficient. Both hosed disconnected, garden hose to inlet, bucket to catch outlet indicates pump is moving water through engine.
6. Both radiator hoses get hot, radiatir gets hot in all places I can touch.
7. Engine seems to run smooth and no coolant leaks that I can see.

Vehicle history (per previous owner):

1. It overheated once, blew coolant out overflow. Did nothing to permanatly address condition, supposedly never overheated again. (But temp gauge wasn't working when I got it.)
2. Uses more than its fair share of oil.

My actions to date:

1. Flushed twice (Prestone Radiator FLush).
2. Replace thermostat 3 times. Latest is original GMC part.
3. Replaced coolant temp sensor and temp swith.
4. Verified some coolant movement through radiator.
5. Verified some coolant movement through engine - pump apparently working (per procedure above).
6. Replaced heater blower motor (wasn't blowing heat when I first got it).

What "the experts" tell me:

1. Water pump not moving enough coolant - blade broken off of blades worn down.
2. Radiator partially blocked.
3. Head gasket blown or improperly installed (but yet engine runs smoothly and no recent gasket replacement).

Can anyone help?

try the pump its cheep and easy or take it off and remove the backing plate and inspect and if u plan on keeping the truck just repalce the rad anyway