1999 Jeep Grand Cherokee won't stop Overheating Please help?

you should have done a cheap and easy compression test before replacing parts you didn't need , first check coolant level and see if fans are coming on when it gets hot , if heater blows hot then cold you have air pockets , if temp. gauge is temperamental it could be the thermostat those are cheap and easy , is there white goop on the bottom of the oil fill cap ? is the exhaust whitish ? Is your compression reading below 100 ? Is your head gasket blown ?

Sounds like a restricted radiator to me, it can cool the slow stuff but when the speed comes up it can’t transfer enough heat. One of those heat sensing guns can show you the hot and cold spots on a radiator.

Does it overheat when the vehicle is moving, like driving down the road? Or when it's idling, like waiting at the bank drive thru? If it's overheating when moving it's a coolant circulation problem. Thermostat, water pump, clogged radiator, clogged cooling system passages in the engine, collapsing lower radiator hose. Sometimes the lower radiator hose will collapse when you rev up the engine. An old hose can get weak and the suction from the water pump can cause the hose to collapse and restrict the flow of coolant. Make sure, if equipped, the serpentine belt routing is correct. If it's not the water pump may be spinning backwards. If it overheats when idling it's an airflow problem. Front of the radiator clogged with debris, dirt, leaves, etc. Clutch fan not working properly or the electric radiator cooling fans not coming on. Lastly, a worn timing chain or knock sensor problem will cause overheating because the valve timing will be retarded (late). Other symptoms of that include lack of power, boiling over when you turn the engine off, and possibly late transmission shifting. Hope that helps! Jim