Well there is what sounds like a lose exhaust bracket rattle when i start it, i didnt think it was related at all, but who knows

 

Sometimes a clogged cat (which is very common on these cars) can cause the feeling of a transmission going. Perhaps you can take it to a shop and have them diagnose the cat?

Also there was an extended warranty on the cats to these cars, so a dealership could possibly replace it for free.

If it is bad, and the dealer won't replace it for free, you should then look at ZZP Downpipes with a Cat

 

Does your check engine light come on?

 

A clogged cat won't always throw the P0420

 

I would definitely look into the cat as being the possible problem. That and service your transmission, and see if that helps anything. If those two things do nothing for you, then start saving for a rebuild.

 

Very true! The cat on my mom's Bonneville was trashed, and it NEVER threw a code.

 

It took two mechanics and two weeks of driving, before i blew a whole in the intake, to get a new cat on. Runs like a top! Thanks for all your help!!

 

u blew a hole in ur intake manifold? and they said it was because of a bad cat?

 

Should have taken one read of my post how it's possibly the cat for you to get the cat checked :p

 

I would of guessed torque converter, i was way off.