I need 700r4/4l60 help

[406 Q-ship]

Thanks to all that give a helping hand with this issue. It is kind of a bizzare one too me.

I put a rebuilt 4l60 out of a 1991 or 92 G10 GMC van in my 74 C10 P/U. It worked great, shifted good but soft like the typical OEM 700. So I installed a TransGo Performance shift improver kit, all was well at first except for a sometime sticky TV valve that I fixed, well over a short time the trans started to act up when cold with the 2nd to 3rd shift. Now this is not the typical 700 2nd to 3rd flare, it will completely free rev between gears for up to 1 or 2 seconds. The free rev only takes place sometimes when cold (rarely when warm after sitting) once it cycles through the gears the trans shifts fine. On the first drive after sitting, it shifts from 1st to 2nd just fine, it runs up through 2nd then as it shifts to 3rd it comes out of 2nd and goes into neutral for a second or two then with a bang into 3rd (unless I lift off the throttle to minimize the bang). Once the trans has done this morning ritual it will shift normal. I think it might be a sticking valve, but which one is the question.

I appreciate any help on this, I will be doing a service on it soon and now will be the time to look at anything in the valve body.