Tranny down shifting check engine light

I have the 4t80e transmission in a 1995 Aurora. I just put a motor and tranny back in the car. I had bought the car from a lady with a cracked block (froze) so I had to replace the motor. She had also told me that it would not shift into 3rd gear because somebody that tried to steal the car did something to it. When she took it to a transmission shop they told her it was the shifter cable? When I had the transmission out I replaced the shift solenoids, speed sensor and the TCC. When flooring the gas on the car the car shifts at 6000 no problem. When I am driving it normal the shift normal sometimes with no check engine light. When I take off from a stop sign the next time it will not really shift from 2 to 3rd unless I accelerate then let off the gas then when hit the gas again it will down shift again into 2nd gear and the check engine light will come on. The light will come on then go off depending if the tranny is working or not. Any ideas? I heard maybe modulator? Anybody know where that is located? Thanks in advance for any help you might be able to provide?

hey
i have a tranny that was shifting out of gear then wouldn't go above 2nd (computer does this to protect the tranny) - turned out there's a screen between the two solenoids that just floats around after it ages because it's held in place with an rubber o-ring.

Replacement kit is available with aluminum L-bracket to hold a new screen in place. Comes with solenoids because the screen bangs on the solenoids.
Total cost $330

Then it started slamming into reverse : my transmission guy found a metal shaving stuck up in the valve from manufacturing...stuck the valve open and the pressure was too high.
Cost: $120

Check the line pressure.
Find a good tranny guy.

s