Cooling system issue on 92 Chev

Have a 92 Chev K1500 with 5.7, overheating issues, replaced radiator, thermostat (2). Runs around 100 degrees without a thermostat but when i install one it pegs out. Heater lines are open and working fine. Any idea what to look at now?
This engine probably needs a good flushing out. It isn't good to run without a thermostat as the back of the engine will be hot and not show it - engines don't like that. The bypass of these cooling systems is the water pump itself and if it can't route the average engine's coolant temp to the thermostat it won't open on time but would eventually and be erratic. Not common but this could be the water pump, crud in system that has to be checked out. These engines need coolant changes regularly and some of the ones GM used Dex-Cool in didn't hold up the five years that they said it would.

This may also be in the club of the 5.7s that had intake gasket that didn't last,

others agree with you, just went and bought a new waterpump, figured out that it is not functioning all the time and should be the cause. Shame they don't build them like they used too, just replaced with the new rebuilt two years ago. Thanks for the advise.
Rick
It may not be the water pump but this is a bit strange and if the impeller isn't up to snuff this could happen. Never used to as this engine in it's basic format has been around for decades on end but somehow when they call a Chevy 350 a 5.7 something happens

BTW - the water pumps spins and pumps the whole time the engine is running - not just when the thermostat is open. When thermostat is closed, part open and wide open it still needs to circulate coolant inside the whole engine to keep it a uniform temp. Without thermostat the pump just pumps a too high % of it's ability right at the radiator without the mixing going on which is what the by-pass system is all about.