one way to rule out the clutch is to go about 30 mph… put it in 5th… and slam the gas… if the rpms climb higher then the car is accelerating… the clutch is slipping… you need a high load on the clutch to determine slip… I had a similar problem to yours… car wasn’t pulling up top… I misdiagnosed it as a clutch… but the probably causes have all been mentioned in here already…
manifold cracks, boost leak, knock sensor etc… i’d put money on the fact it’s probably pulling timing for some reason.