Did you do some engine work to this truck like a valve job just before this started? Could it be that your guides were damaged when they installed them. If the machine shop has a defective installation tool and didn't go back and check the bore, it could effect more than 1 valve. As the engine warmes up expansion and contraction of the valve and guide will be differant. This would explain a 20 to 40 mile drive before problem occures.
