tranny issues

First off its a 1994SC automatic. Shifts from first to 2nd are messing up. It will shift from first back into first. It will act like its going to shift into second and just go back into first. It usually only does this at WOT or when I am moving faster than normal, also only does this once and a while not every shift. I took it back to the guy that rebuilt it he hooked it up to the machine he got no codes and said everything on the tranny is fine. He thinks its definately an electrical problem. My question is does this make sense?and what controls my shifts is it the EEC?

First to First Shifts at WOT

Leaking seal on pump and/or wrong spring pressure in valve body for 1st to 2nd valve/piston/or sticking piston in valve body due to burrs. These are just guesses. Best check with someone that rebuilds these with the J-Mod and what not and see what they have to say Bottom line is, something is interfering with the shift, and it can be fluid pressure for a given circuit, or a mechanical/pressure problem. The J-Mod changes the pressures for given circuits, and it's possible that you need to install a really good cooler.

The tranny guy checked over the whole transmission and scanned it no codes or nothing. He even drove it around for about 30mins and still no codes. I remember when my tranny first went before the rebuild it did the same exact thing