A fine lens it is. Check the gold contacts on the back. Sometimes cleaning GENTLY with a qtip and alcohol will help. Other than that you should call Canon and see if they will check it out for you. It's worth it to get it fixed even though 14yo. I bet it's the contacts though. They don't last forever.
I've also see this happen with dirty battery contacts since on some cameras the AF motor gets its power directly from the battery. However since you have tried it on several bodies, I would say it's likely the contacts. But it could be something internal that needs to be repaired.
+1 A pencil eraser also works...
WTF? do you want him to ruin his lens?
The pencil eraser technique has been around for a long time... it won't necessarily ruin his lens, it might not be a great solution though. It's oxidation they are talking about. Check with a electrical shop and pick up a cleaner specific for this and use gingerly on the on contact surface.
I don't mess with this stuff personally, I'd just send to CPS or find a third party repair shop. Here in the Bay Area there are only a few that can tackle the old stuff.