Started overheating yesterday?

My 96 3.8L V6 Mustang started overheating yesterday. I put a new thermostat on because the temperature would fluxtuate between the L in normal and the A in normal. The new thermostat did not solve the problem, so I removed and checked the hoses, flushed the radiator and ran water through the block. No debris came out of either the radiator or the block, and the fluid is flowing unrestricted from the reserve coolant tank into the radiator. There are no fluid leaks, but the engine is still running hot. Once I realized the thermostat didn't fix the problem I removed it completely just to see if maybe the new thermostat was perhaps faulty. After the last test drive I took it on after having checked the hoses and flushed the radiator there was smoke coming from the oil dipstick and a bit of oil spewed out from the dipstick, the engine was running hot during the drive however and I'm not sure whether or not that was the cause for the oil. I should also note that even when the engine is running temperature is running very near to the red line if I turn the heater on only cool air blows out from it. The engine has 139k miles on it, but I have replaced all the pullies, water pump, ac clutch, spark plugs and distributor cap.