2000 accord, ac works great for about first 30 minutes then starts blowing cool to warm. stopped at pep boys on sat and they said freon good but needed expansion valve...could they be right? how can I tell if they are telling truth? my uncle has gauges but never worked on honda before. thanks need help as really hot here in texas

 

When you turn the AC on, the condenser fan (One of the two fans behind the radiator should turn on. So have some one check if one of the fans come on when ever you turn AC on. If it does not, then the hot refrigerant going through the condenser doesn't get a chance to cool. Therefore initially you get cool air and after a while it gets hot. Again if you are continuously driving the air draft should take care of the lapse on the fan's part. The thing that i am theorizing should be more prevalent when you are stopped idling and the AC is on where the condenser fan is not working.
I had a similar problem on my 93 Accord, where the condenser fan won't come on when AC is turned on, but when the engine temp reaches the set point, both fans will work. So I knew my condenser fan is good. I didn't know what was at fault and the mechanic suggested changing out the computer module etc and I was not ready for some big expense. So being an engineer, I rigged up my own little switch in the dash and reconnected the condenser fan. So every time I turn the AC on I flip that switch and I have a LED light that lights up to tell me there is power to the fan.
Any way I didn't mean to bore you with what I did, but thought I may let you know that the fan not running caused similar problems as you had mentioned.

 

Like ramachan9 says, except in 2000 Accord, BOTH fans are supposed to run with the AC.

 

Is the compressor refusing to engage when no/low cooling occurs? If clutch is engaging, and cooling is low, then expansion valve seems likely. You would need to check the AC system pressures while this problem is present to be sure. If compressor is refusing to engage when low/no cooling occus, then problem may be clutch gap.

This problem has appearance of excessive clutch gap. If clutch gap is too large, the clutch coil field strength is not enough to close the clutch plate and drive the compressor. It's easy to test if this is the problem (but be very careful!). Get a hefty stick of wood and rap the face of the clutch. If it engages and cooling begins, then too large clutch gap is confirmed. Solution is to remove clutch plate and remove 1 shim from behind the clutch plate.

good luck