4L60E harsh shifting help??

Looking for help/ a point in the right direction on my 01 silverado with a harsh shift It shifts hardest from 1st to second and then shifts smoother every shift after that and if I pull over and shut it down for a few minutes it shifts smooth for the rest of the drive and comes back the next time I drive it. I bought it knowing it had this issue and got a decent deal because of it but I am now a little bit challenged on what may be actually causing the harsh shift… The transmission was rebuilt and only has about 20,000 miles on it so I thought I would be safe from finding any major damage this early after a rebuild. It does have a shift kit installed in it but the previous owner says that it shifted smooth/normally right after installation and started to shift hard when he attempted to drive from California to North Dakota. I started off by dropping the pan and replacing the epc solenoid and did a fluid and filter change at the same time. I was excited to try her out again only to find it still does the same thing. I then read that maybe it is the throttle position sensor but dismissed that when I plugged my obd II reader in and watched the throttle position and saw that it was working fine (Same thing with the MAF sensor). I ended up buying a corrected valve body with the sonnax tcc regulator valve fix thinking that would be the issue and threw it in the and to my surprise the harsh shift came back 30-40 miles after… Only codes I have in it are P0332, P0327(knock sensors) and a non confirmed P0748 (transmission fluid pressure solenoid – Electrical). Any guidance is greatly appreciated, My next attempt to fix will probably be replacing the solenoid wiring harness or replacing the 1-2 accumulator springs to the stock ones. Thanks