Here's the problem
 

This problem just started. At full throttle at around 6000 rpms the car should shift from 1-2 but instead it drops to about 4500 rpms then slowly shifts. The simple answer is the trans is sliping but at low throttle and even 3/4 throttle it shifts perfectly. I dont know if it does it from 2-3 I havent hit the highway yet. Do you think its just the trans slipping or could it be something else?

What was the last thing that you did to the transmission? I would check there first.

The only thing I did to the Trans was the slp mod and I took that out to see if that was the problem. The fluids full and everything. I never had a problem before.

How many miles are on that transmission? Did you ever change the fluid and filter before and how long ago was it?

The Trans is old and I'm sure its just wearing out but I thought it was weird that it doesnt slip in any other throttle position.

I'd say it's time for a rebuild but that is just my opinion. Good luck.