Tranny slips and then it doesn't??
 

Ok, I put a 6.0 ltr in my 99 Formula. I have a 4l60e w/ a tci 3500 stall. After the install of everything it all ran great. Shifted fine through all gears. All of a sudden one day it just started slipping at the stall point, right around 3k rpm's. Perfectly fine anywhere below those rpms, pulls great, does a burnout, shifts fine. But as soon as you give it gas and it stalls, it slips. Then just as sudden as it started slipping, it stopped slipping. It slipped for about a week, now it was perfectly fine. Until last night, I went to the store, everything fine on the way there, then when I left, it started slipping again. Anyone have any ideas? Must be electrical right? Tranny was rebuilt a few thousand miles ago.

Needs to be tuned for the converter.
What you've done already is hell on the 3-4 clutch, hopefully it'll be ok.

You sure your car is not missing? People get that confused a lot of times.
Trans cooler?

I have a full tune. The trans. fluid looks and smells good. Brand new TCI 3500 stall.
Is there anything electical that could cause a tranny to slip??? I took an alarm out of it and this happend 2 days later. Drove it for 5 days slipping, then it was fine for 4 days. Now it's sitting in the garage til I figure out what is going on. Someone told me there could be a seal in the trans. that could be blown and is acting like a flap, catching and uncatching. ??

It's not missing, I did have a miss, but my tuner fixed that. The motor runs perfect. No trans cooler, but it doesn't matter if its hot or cold, it will slip and not slip either way.

You need a trans cooler asap first of all.

Yes I know, but I'm just driving normal and it's very cold out.