Drivetrain Looking For Transmission Info

Well I got my car back last week ended up had a bad injector now when I was out cursing this weekend and od started slipping (might have started awhile back when the motor was running bad and did not realize it). I dropped the pan back in feb and changed the filter and the fluid that I lost reason for that was because I had a transmission shutter I put some shudder stop and it has been doing fine till now. Im going to ride to a transmission shop sat and let them take a look at it but I figure since its 17 years old with 85k on it and will probably need a rebuild and would like some suggestions for up grades they can do while they are in it. I would love to put in a 5 or 6 speed but cant since my wife cant drive one. Just looking for some suggestions on if it can be beefed up so I can ask the shop if they can do it. the car is pretty much stock and I just cruise but would like to do a few things in the next year or so it would be nice to take car of anything now while im at it.

Thanks for the info. I drove the car today and it acted up some and was pinging when I would nail it. After I got were I was going it sat for about 15 min then I drove it for awhile and the car did great no pinging or slipping then later we took it to the in laws and it did it again so I was wondering if that meant anything since its not all the time and when it acts up it does the same two things.

Thanks guys i will look into that also im not sure i would say its slipping to 3rd more like its kinda the motor has a miss i might be describing it wrong though.