2000 chevy 3500 express slips out of o.d.

this van slips out of o.d. will not go back into o.d. unless i pull over and turn off for 20 seconds. after that it is back to normal. any ideas.
thanks

Welcome to AF.
What transmission?
How many miles on it?
Fluid level and condition good?
Any check engine or transmission lights on?
Is it dropping down to 2 gear?

You need to get a scanner on it and get codes.
post back codes no.
MT