What exactly do you mean you lose the gear? Does it firmly click into gear but you have nothing or it wont allow you to go into gear?

Its odd that all the gears your having an issue with are on the same axis. doubt its it but check the cables under the hood and makes sure they are still clamped to the bracket. Do not confuse this with the shift lever on the trans, the bracket is to the rear of the shift lever.

The fact that it works when its cold but not when its warm leads me to believe the problem lies elsewhere. Check the cables anyway.

Terry