Auto Trans not shifting into 3rd

I purchased a 94 Astro, automatic trans with the multiport injection. When the engine is cold it will shift normally. When it warms up, the trans doesn't shift up into 3rd or overdrive and when you stop, it doesn't down shift to 1st. I can manually shift into 1st but it won't go into 3rd. I changed the trans after losing reverse, but the one out of the bone yard is doing the same thing. Someone told me that there is a problem with the injection system but wasn't clear on what the problem is.

You may need to set up the van on a scanner capable of reading transmission codes and operating the shift solenoids. A Tech II can perform the tests, or you can measure the cold and hot resistances of the shift solenoids through the wire harness.

I'm suspecting a shift solenoid may be overheating and opening or shorting until it cools. The fact that it operates relatively normally when cold would tend to eliminate any mechanical problems. Since you can force it to first via the control spool it sounds like a hydraulic problem in the valve body.

There is a remote chance the line pressure is not being modulated up when hot, but that's still a function of the PCM and valve body PWM solenoid controlling the pressure.

first mistake buying 1 at bone yard or rebuild your org...

yes i got the same problem before with my Chevy. I bought a transmission for really good price in the auction. They didn't have parts but i bought a vehicle for 300$ with non repairable certificate which means for parts only. So if it helps for you I can give you a link http://www.easyexport.us/?a_aid=4bead65889ce2
Also i still have some parts available if you need that :)