93 Sunbird temp needle bouncing.

I have a 93' sunbird LE with a replaced 2.0L engine. First engine locked up, new one has about 80,000 miles on it. The temperature gauge bounces all the time, it literally redlines to the point that I can smell antifreeze through the heater vents. It redlines for maybe 10 seconds then drops down to 1/4 of the way up.

Its constantly moving from 1/4 to 3/4 and sometimes higher. Replaced 3 thermostats in past year, nothing was wrong with any of them. Radiator has been replaced and the water pump was replaced not too long ago. There are no external antifreeze leaks, but yet it seems to lose little bits of coolant over about a months period of time. The engine gets very bad gas mileage and the check oil light is always on, even with the oil at full. Sometimes the engine is a complete pooch and wont acelerate very well at all..

The heat is always constantly getting cooler, then hot, then cooler again. Coolant will be full and the heat still wont heat up for nothing, even with the temp gauge being at over halfway. Im thinking head gasket, or maybe even a cracked head. Maybe the coolant system needs flushed, that might explain the heat problem but what about the temp and bad gas mileage?

GIVE up on the thing and save up for a better car. Pontiac should give 3000 grand to everyone with a sunbird b/c there the biggest pieces of you know what and there cooling systems fall apart quicker than civics bodies.