1995 4L80 no first gear when hot

I have a 4L80 transmission in a 1995 Chevrolet P30 chassis. It has 130,000 highway miles. It recently stopped shifting into first gear when hot. It works great when cold, but as soon as it warms up, no first gear. It will not shift down to first at a full stop, it starts in second gear. It will not down-shift to first on the detent. It will not drop to first with the selector in position 1, even as you slow to a stop. It only goes down to second gear. All other gears work OK, including 4th. The speedometer works OK.

I changed the throttle position sensor, and it got a new filter and 6 quarts of synthetic transmission fluid 50 miles ago. The fluid had been changed only 30,000 miles previously. The problem did not change.

I do not have a scan tool to look for codes. I just did a manual check for stored codes. All I got was code 12, which I think means the diagnostic system is working.

Any ideas?

i have a 4L60e and had the same problem. it turned out to be the shift siolind A. witch is 1-2 shift. my truck only would start in 2nd. i know this is not much help but its a start.