Engine Performance problem
2001 Daewoo Nubira 4 cyl Two Wheel Drive Manual 110000 miles

I recently purchased this car from a dealer with 10800 and it had a small accel problem. If I pressed the gas more than 1/3-1/2 the engine would bog down. As I drove it this problem got worse and the car developed a hitch and power loss only on initial few mins. It worked out of it as the car ran fine, but with same issue with the accel problem. Eventually it started doing it continously and started making a strange hissing noise. I parked it and asked a friends husband to come down and take a look at it, he's an ASE mechanic, and he told me it was the valve cover gasket. Well, I replaced the gasket, the spark plugs/wirtes and cleaned the wells. It helped alot for a few mins, but the hissing continued. Now it just will not turn faster than 2500 RPMs and refuses to accel at all. Any help will be greatly appreciated.