2004 s-10 blazer transmission problem

2004 Blazer, 4WD, automatic transmission about 11k miles.
Very intermittent problem. Doesn't want to shift from 1st to 2nd gear. All other gears seem o.k.
Engine revs up....doesn't shift, feels like clutch pedal is depressed, of course there isn't one. It might happen several times a day or only once or twice a week. This occurs when the engine is hot or cold.
The dealer can not duplicate the problem. They changed the fan clutch, didn't help. One other thing....no engine code.
Thanks for any help you can offer!

Back from dealer again. Could not duplicate. They kept this blazer for 2 days and only drove it 11 miles. And they wonder why they can't duplicate the problem. Why do service departments have to "duplicate" the problem? Why won't they drop the pan and do an inspection?

Probably not related, but I noticed the "Service Engine Soon" lamp does not come on when key is turn to run position, before starting vehicle.

I don't think my SES light comes on when I start it either (98 Blazer). I know it works, because it comes on when my transmission starts doing the p1870 code thing (http://www.automotiveforums.com/vbulletin/showthread.php?t=240702).

The newer transmissions are electronically controlled by microcontrollers and sensors. They need to duplicate the problem so they can get a capture of the events using a diagnostic scan tool. It shows them engine RPM, what gear it's in, and all kinds of sensor outputs, so they can tell why it's doing what it is doing. If it's an electrical thing (like a sensor or solenoid going bad), they can't tell by looking at the transmission with the pan off. I worked at a Toyota dealership in the service department (computer jockey) for 7 years, and I saw alot of "could not duplicate" repair orders. If customers come back enough, they hand it off to the "professional troubleshooter" (the service manager, in our case, who is an ASE Master Diag. Tech and a Toyota Master Tech). So, the moral of the story is keep being a nuisance by bringing it in everytime it acts up, keep an eye out for what you're doing when it acts up (like going uphill/downhill, the RPMs, ambient temperature, etc.), and they'll eventually get it fixed. If the car doesn't do it while they've got it, chances are they'll never figure it out scientifically and just start changing parts to get you to go away happy.

Thanks for the info. Maybe I'll try a different service department.