If you're sure it's in the top end of the engine, you'll need to pull the valvecover (will require removing the upper intake plenum as well if EFI) and crank the engine over (either by hand or with an assistant turning the key, make sure you disable the ignition system and pull the spark plugs first, as well as the fuel pump (simply pull the fuse) if it has an electric one.) and see if the valvetrain is working as it should. If the followers are wobbly (side to side or back and forth) you've either got excessive cam or follower wear or a valve spring or lash adjuster (often called a lifter on these cars though it's a bit of a misnomer) that is either stuck or weak. You'll need to use a dial indicator to check for cam wear, follower wear will be indicated by deep grooving or scarring in the surface of the follower (the surface that rides against the cam, you'll have to remove them to check them, just do them one at a time so you don't get them mixed up), and lastly you can check your valve spring pressure with a gauge (find a tool rental store or a machine shop that will rent/loan one) and verify free movement and good oil flow to the lash adjusters.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
