99 Venture 143k. When you drive it 12 or so miles the transmission shifts itself into neutral. Once you pull over and shut it off, you can then start it back up and put it into drive. It will drive as if nothing happened until you go another 12 or so miles. Then it does it again.

Any thoughts?

sounds like a plugged or restricted filter,,,,,just change it,,,not a money wasting flush !!!!!!

Could be poor electrical connection......

What do you mean it 'shifts into neutral?' Is that another way of saying it starts to slip to to point where it won't move at all? When that happens, does reverse work?
Sounds like tranny overhaul time to me.