Where to look for automatic transmission air leaks?

I don't actually see a leak, but read that if it ingests air that will cause it to lose pressure. Since it shifted just fine after the mechanic tested it and put it back together, then slowly started slipping more and more as it was driven, I thought this could be the cause.

I would agree except that the transmission worked perfectly as I drove it away from the shop and only started shifting irregularly after a period of driving. Something (heat, air, leakage?) Is causing it to fail after a period of driving. I see also that the valve body gasket could be a source of leaking. Would a leaking valve body gasket cause these symptoms?

Okay, so not air. Drove the car again this morning and same thing happened. Changed gears flawlessly for about 10 minutes and then started not finding second as easily. After about 20 minutes, second would only kick in at very high RPM and roughly. I haven't really tested 2-3. Solenoid? If it was bad I wouldn't think that it would work cold and not hot.

I did not write the last clearly. I haven't tested the 2-3 shift solenoid. Could it be a solenoid problem? If the 1-2 shift solenoid was bad, I wouldn't think that it work well cold and not hot, but I'm not a mechanic. I mentioned that it just came out of the shop, although not a transmission specific shop. It is doing this after the mechanic looked at it, so I'm looking for diagnosis assistance or at least second opinion. I think the mechanic's next step will be to replace the 1-2 shift solenoid. I'm not in the U.S. And will have to order all parts which will take time, so I'd like to get the diagnosis right the first time if possible. So far none of the experts here have provided any useful diagnosis assistance except to tell me that I'm not right. How about what is right (I don't have access to a transmission shop)? Maybe some steps to take in diagnosing?