2000 astro won't downshift

After sitting for 5-6 hrs and then starting up to go somewhere the transmission works perfectly. After 15 minutes or so of stop and go traffic the transmission will bam into 2nd gear from a standstil accelerating. Then It will not downshift to first gear at the next stop. It will start in 3rd and stay in 3rd till you stop and shut off the ignition for 10-20 seconds. Then the trans will run all day perfectly until you shut off the truck for 5-6 or more hours.
A "reputable" trans shop had the truck for 2 weeks and replaced the solienoids and the ignition switch wiring harness as the codes indicated. no luck. An aftermarked security system had been installed when the truck was new and was wired into the ignition switch wiring so they uninstalled the security.. Now i'm out the security and 300 or so bucks and nothing. Can anyone help?:frown:

Is there a constant "CHECK ENGINE" light while driving?

Did the trans shop happen to remove the valve body? Inspect the separator plate? Check the accumulator? (A good trans shop should have recognized the symptoms.)

Is there a constant "CHECK ENGINE" light while driving?

Did the trans shop happen to remove the valve body? Inspect the separator plate? Check the accumulator? (A good trans shop should have recognized the symptoms.) Thank you for coming back to me. The engine light stays on and has since this situation started. Did they what? The trans shop and the Chev dealer both said they checked everything.. duh I know what that means as the prob stil exists.. they did'nt check something or they could have fixed it. My prob and the rest of the "Publics" prob is we don't know what to ask. You mentioned the plate and the valve body.. do you know what to look for in the valve body specificaly to this prob and what do I look for in the plate? Your appreciated in advance for helping me.. thanks

Time to find another reputable shop. And not all dealershsips have transsmission-trained technicians, even though they might try working on one anyway.