Motor mounts, fuel delivery (pump, carb, lines, filter). Electrical. That's a lot of stuff, I know. I lean towards motor. However you did not say engine was missing, sputtering, dying, choking, backfiring. Can be misfire, crossfire, detonation......plugs, wires, points.....TIMING CHAIN slipped..
You mean clutch when you say slipping feeling, right?
You will have to drive more, be aware of what it does and give us a better feel. Too many possibilities right now.
Check motor mounts first. Front/side and tranny mounts, then timing, then look for weak spark so as to start eliminating systems. If it seems serious, check compression.
