My Chevy Venture 1999 when I drived the temperature was normally, but when I stay the temperature was hot. I changed the termosthator but the problem is continually.

Check coolant level,also fill reservoir with coolant to cold fill line, check radiator for coolant flow, check electric fans for operation. alot of times when a vehicle stays cool when driving is because the air is flowing through radiator fins and the waterpump is circulating the coolant which keep it cool. when vehicle is at a stop or stand still the coolant is not full and will not flow through the radiator nor will the air cool it. your likely cause is low coolant in radiator,

Check the coolant reservoir and the radiator for proper coolant level. If low you probably have a leaking intake gasket or possibly a head gasket leak. The intake leak will be visible on either end of the intake manifold, located at the very top portion of the engine. The head gasket usually shows up at the rear head, nearest the belt. You could have a water pump leaking and this would show coolant on the ground under the right front of the engine, more so when the engine is running. They are very common for all 3 problems.