cuts off

I was wondering if someone could help with a strange problem.
My dad has a 93 f150 with a 302 that runs great going down the road and at idle while it is in park. When you put it in reverse it does ok but if you put it in drive it will cut off unless you hold the gas a little. We were told to look for a vaccume leak but all the lines seem to be ok,carb cleaner sprayed on them has no effect at all. Does anyone have any other clues what to look for.

Thanks