Is my engine on the way out?

I have a 1991 z24 3.1 with a 5 spd. When I start it and move from a dead stop it wants to die. My engine is running/idling rougher by the day. I found a spark plug out of the engine yet again. Why is this happening? Oil accumulates on the top of the engine and the oil level is low constantly.

How many miles, last tune up, fuel filter, oil change, air filter, pvc valve, checked for vaccum leaks, ????????? BE more specific SPARK PLUG OUT OF ENGINE??????? BAD OR BACKING OUT OF THREADS????????

The plug backs itself out of the engine. The pcv valve is the only thing i have not checked and there is a vac leak behind the engine. This seems to happen repeatedly though and with different plugs/cylinders.