GM 307QLV 200HP Sterndrive 1972 Chris Craft Lancer 19'

GM 307QLV 200HP Sterndrive

I just picked this boat up a couple weeks ago and I'm new to boating. First few times out it was great, planed out well and would hold 3200-3500rpm cruising speed. 3rd or 4th time out I started hearing an odd thrashing sound from the upper unit(I think). I'd slow down, ease back into it and it seemed like nothing happened for a few minutes then the rpm would go down maybe 200-500rpm with a bit of power loss (obviously). Went out yesterday and just as I was curling into my bay there was a pretty solid grind again. Being close to home already I idled back to the dock where it stalled roughly 15ft from and unsure why. I'm unsure what's going on or what would cause this. Went out today and I can't plane out, vibrates pretty good and I can only get up to around 2100rpm without sounding like a bag of s***. Any advice would be appreciated. Previous owner had motor rebuilt, has 204hrs on it now, had a larger cam put in as well if that makes a difference