95 V6 Misfire

So I have a 95 Ford mustang v6. It had been over heating for like a week before I was able to determine the cause of it was a radiator fan motor. When it would get hot the lifters in the valves would start to rattle a lot and then it started getting really bad a couple of days ago when my fiancé's step dad took a circuit tester and connected to our battery and then stuck it into our spark plug wire while the car was running... And ever since then the car has been running really different. When it starts it tries to die a little after like minutes even seconds sometimes and I can keep it running if I have my foot on the gas but its really sluggish it doesn't want to speed up, it misfires, and it tries to die any time I come to a stop. Honestly I don't know if these problems are from what he did or from the car over heating so much..

P.S. I found that there is like an oily water substance coming out of the intake manifold down into the air filter.. I really hope this is fixable so please help me!