poor acceleration with loud hissing (archive)

My 86 735i just came down with a sudden case of bad acceleration. It is so bad as to make the car unsafe to drive. No (automatic) transmission slipping it evident and it shifts firmly into each gear change when it can get up enough speed to warrant the next gear. Revving is limited to about 3,500 to 4,000 RPM in neutral or under load. There is also a loud hissing noise that I can't pinpoint with the car stationary. When trying to accelerate, the hiss gets louder as the RPM's slowly climb. There seems to be a slight exhaust leak from the manifold near the #5 & #6 cylinders. There's also a leak I've known about at a welded coupling under the right front seat area. This came on very suddenly, according to my wife. I drove it the day before and had thought to myself how well the old beast was running!
My first guess would be a clogged cat but would that come on so suddenly? I was going to drive from East Tennessee to Atlanta this weekend but this may be out if I can't find a simple fix. Any help will be a great help! TIA! And Happy Holidays to all!

Follow Ups:

Could be a clogged exhaust if your A/F...... Bob745 11:54 12/22/2001 (2)
- Re: Could be a clogged exhaust if your A/F...... Tom B. 86 735i 13:18 12/22/2001 (1)
  - Re: Could be a clogged exhaust if your A/F...... Bob745 14:53 12/22/2001 (0)

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