Engine low power

A few weeks ago my car started vibrating when I get to about 50 and try to accelerate. The other morning I left for school and about a mile down the road it quit accelerating. I had to push the pedal to the floor to get it to 30mph. I assumed it was the cat. Converter. I had my boyfriend cut it off and put on a flex pipe (no emissions testing here). I've already replaced the air shaft sensor, coil pack and ignition module along with the alternator, steering pump, water pump, spark plug wires. I dont have the cash to just take it in. I'd rather just start with the easy obvious things. Any ideas?

I had exact same problem with the 1998 buick park avenue that has 86k when I bought used; after drove for 5,000 miles, it accelerate short distance than stop but engine still maintain same rpm. I checked tranny fluid, was ok. I drove it the next morning but the car doing the same thing. After cool down for 15 minutes I made it to the auto repair shop. It was a major shock it to my wallet; rebuilt tranny set me back $2,500. Buick is known for tranny going south once reachs over 75K to 100k miles. The previous owner must have known this problem.