1991 Jeep Wrangler. 4 cyl 5 speed, 105,000(ish) miles. Seen limited usage in the past 10 years. Throwout bearing started leaking, researched the repair and told my sister to take it to a trans shop (relatives dealt with this place and they were highly recommended) After it came back from the shop, the cat converter was rattling (it broke internally- this has happened before and I do not fault the trans shop). I showed my sister the price for a new cat and exhaust system and ordered the parts. Her husband drove the jeep and said something was wrong with it-it didn't have any power. She drove it for 2 days without a problem. I drove it and the problem showed up. It was much quieter, reved ok when sitting, but when you drove it, the power went away and there was a hissing sound( like the sound of compressed air coming out of a blow gun) from under the hood. I found the headpipe (pipe between exhaust manifold and cat converter)was loose. I guess it was loosened for the trans work and wasn't tightened. I thought the cat plugged up and the exhaust was trying to squeeze out by the manifold. Got the parts, removed the exhaust sys, started and revved the motor- seemed ok. Install parts, took it for 3 mile ride flooring it up a hill. Ran fine. My sister drove it home, doing 80 on the highway, said it ran fine. It sat most of the week outside. She tried to take it to work today and said the problem was back. I haven't looked at it yet, and we need to get it to my mom's house (where all my tools are at). Thoughts? Can you think of something I missed. Is the Jeep not happy being parked outside of a nice big garage? Sorry for the long winded post, Im trying to give as much info as I can. Magnaflow cat converter and Dyno max(?) cat back exhaust system (the one with a $75.00 rebate). When I took the old cat off I shook it and it was ratteling-didn't cut it apart. This is the 2nd time a cat did this on the Jeep.