The power supply is definitely the place to start.

I've heard of this happening because its parked at a floor and loses DZ input, at which point the car thinks it's not at a floor, so it runs to go find one. I think in this case DZ magnetic switch was replaced, because the internal contacts weren't conducting well.

That's a great idea. I'll let him know to check that. Thanks for the input TC7.

Does the car run to a random floor after making a run, or does it just sit at a floor and randomly go to a floor?? Do the lights for car and hall calls light dimly on the cards? If the former, look into the closing sequence. First you get the gate signal, (X5 I think), then you get the door lock (X8 or X8a), then you break the DCL. I have seen x5 go high and then go low for a second, (dirty gate switch in Operator), get x8 and then get x5 aagain and this causes the car to run one floor (up if you have 4 slot cards and down if you have 3 slot cards.
If the latter, look for a bad ground. The board ground is the center tap on the transformer.

Thanks Johnny. Those are great tips. I will have him check into these.