mysterious charging problem!!

My battery charges fine when I have the truck sitting in the driveway, but will not charge when it is going down the road. My mechanic has checked the alternator (OK) and starter (OK also). When I drive to work I have to let it sit idling for 10-15 minutes and then it is fine 8 hours later when I am ready to go home. And I have to go through the same when I get home, and it is fine the following morning. Mechanic seems to think it is the computer sending mixed messages to the alternator. I know very little about electrical systems. Please Help!!!!!!!!!!
(I have a 89 Dakota 3.9 4WD)

I would change the alternator before I changed the Computer it's much cheaper, thats a weird problem, I wonder if somehow it has some bad brushes or something in it that don't make good contact at higher RPM's ???? Threre are no "signals" sent form the computer to the alternator, the voltage regulator is in the computer but thats about it. So with that said I guess thats somewhat of a "signal" but nothing crazy complecated like digital, it's just like the old school Mopar charging systems, but instead of having the regulator in a box on the firewall it's in the computer. I don't think thats likely to be the cause of the failure since it's not moving ya know? I would change out the alternator it's self, I would also check the grounds to the chasis and the motor, I'm sure it's possible that the engine is torking and disconnecting a ground, well something to try anyway, hope this helps.