Inherited 1990 Silverado w/Overheat problem??

    My dad gave me his 1990 Chevy Silverado(?) pickup (disconnected plow) with a new 350 engine and tranny. He never drove it fast, as he is a very slow driver. Now when I drive it like it should be :), the temp guage shoots up and will climb to the top if I maintain highway speeds with it (65-70 mph). The radiator and thermostat is new, as are the hoses. Coolant level checks out fine....tranny fluid level ok....and i'm pretty sure it's overheating, cause it got so hot it melted the air cleaner gasket :*O  ....hmm...could this be caused by his slow driving habits and not breaking it in right? Is there a common issue with this truck? Maybe the thermostat has to be replaced again?

Here is an experiment for you. Sometimes, even if it's new it can still be bad. Try taking your thermostat all the way out and put everything back together without it. It will take it longer too heat up and might run differently until it does, but this is too be expected. Now drive it just as you normally would and see if the symptoms continue. If not, you purchased a bad themostat, but if so either your radiator is stopped up or the impellor on your water pump.
 
Hope it helps.

Flush it with a flush kit, fill it, replace the thermostat, (go 180 degree if you want) and try it.  It prob just has some build up. 

First thing you need to do is get the engine to normal operating temperature.  With the engine running feel your upper radiator hose.  It should feel pressurized, and be pretty hot.  Then feel your lower radiator hose.  It should also feel pressurized but significantly cooler.  Then do the same thing with the engine running but this time with a cold engine.  There should be very little pressure on both hoses.  After you have performed these tests, report back with the results and I'll tell you what is wrong.

Check the water pump to see if the impeller blades are OK.  If the old water pump was used on the new engine, it could be that age has caused the blades to be corrroded away.  If so, the pump is unable to move any coolant.

I have a 1991 chevy 1500 and was having similar problems. i was sold a new faulty thermostat so it happens. i would remove thermostat, place the thermostat into a pot with water. heat and boil the water, once the water is at boiling temp the thermostat should open letting you know exactly if the thermostat is good. i would also like RonH said i would double check the water pump. hope this helps a little.