i would defiently check the intake plenum and gaskets but if its not that it is a head gasket for sure. those would only be the two problems. the other way you can find out if its a head gasket is smell the exhaust and see what color it is. white smoke of your tail pipe, or loosing coolant through your overflow , take the cap off and rev the engine, if you see bubbles or if it comes out you ll know. A blown head gasket will leave a dark smell in the radiator. And you will have high back pressure coming through your radiator cap. also one of the signs are smoking really bad. If the car is on and you are standing right on top of it you probably be able to see it smoking. Check or have the intake plenum and gaskets checked much cheaper fix if you don't do it yourself. also if you will feel a miss at idle and the idle speed may be erratic. if that'* the case then the intake manifold will have to be removed and the intake gaskets replaced.

I dont understand why you keep "changing" the plugs. Just cause they are gummed up doesnt mean they are bad. If they are new plugs then clean, re-gap, and put them back in.
It appears that you are aware that its not a plug problem.
The shop that worked on your car and "FIXED" it the first time ????? Why not take it back to them and have them fix it right ????

Not to resurrect an old thread, but... after reading thousands of threads over the past 5 years, this would be the third one I have read about, if it'* the head gasket. The other 2 were after the fact. A mechanic just diagnosed it as the head gasket, and fixed it. It was at the same time as a UIM/LIM failure. Out of 10'* of thousands of problem threads, the head gasket is the very last thing to consider.

BTW, Another good way to check for an impending UIM/LIM failure is to check the spark plugs. The detonation with anti-freeze present will thoroughly clean one side of the sparkplug(*). You need some decently mileaged plugs to be sure (probably 10+K miles). It will clean a lean burn plug, and is really obvious on a rich burn plug. (I think I can find some photos if there is any interest.) I did pull the plugs and noticed some cleaning on a few of them, but at the time I didn't know to associate it with a leak.

My wife caught this situation well before I could be sure that the system was losing antifreeze due to the UIM/LIM on her '96. No chocolate milk oil or sweet smell from exhaust. A mechanic (who owns a '95 SE) and I both drove the car and it seemed OK. But she kept pestering that her car didn't "feel right".

Within the next month, she lost a half inch of antifreeze. I did the gaskets and found the upper just about ready to open the flood gates.

FWIW, maybe it will save somebody a hydrolock.