cylinder not firing?

So, heres the deal

I just bought my first '89 Barefoot Nautique model, with an '88 model GM in it, I have no idea what size or model number, but it seems to be running fine.
Alternator and starter were serviced, fuel lines, oil, oil filters, and spark plug cables were replaced, the distributor cap looks new.

The engine ran great for the first two days, and then suddenly sputtered and stalled once when giving half throttle. Now I cannot take it above about 1200rpm when in gear without it sputtering and stalling, although when not in gear it runs great past 3000rpm. I can only guess that maybe one or two cylinders arent firing or something... I get no backfires or any signs that my timing is off. Any help?

you should be able to tell if it is not running on all eight cylinders because if it was running on only six you could not get full power out of the engine no top end. I would suspect possibly some thing in the carb or fuel system. The engine to jump time it would have to sheare a tooth off of the gears because the engine was built with a gear drive on the cam.