You have an issue... somewhere. Sounded off the cuff like a manifold leak, but it recreates as soon as y our car gets hot (and 280 IS hot for a T). If it were carb probs, a 4 would be running lean (and hot).
If it were me, I'd drain the water, pull the head, and check to see if your head gasket isn't either blown or "not punched". as per above. Also to a water flow test on the head with the head off to make sure the rear passages aren't clogged. Remember, low heads don't cool as well as high heads also.
If ALL of that checked out OK, I'd start looking for ugly culprits - cracked head, block, etc... or a cavitation (electrolysis) hole in the cylinder or head... or something.
The head is easy peasy to pull (and you can also check your valve height while there to rule out a flattening cam)... Good luck!
Sus
