If your youtube doesnt work on HTC evo try updating it and if you still have the same issue download VEVO, its a lot better.. also Updating it to 2.3 (gingerbread) makes it faster.. If you are eligible for an upgrade, i would siggest you to go with nexus S .. i had an evo, then I switched to Arrive, then Shift, then NExus S.. trust me Nexus S is the best out there.. Battery life is way better than any other.. videos plays without buffering... 

P.S. Looking at all the customers that I get from ATT i really suggest you stay with Sprint coz the plan is really good and get an EVO or NExus S.. if you want discount on your bill email me and ill add extra discount!! :)