Auto Tranny Problem

This is for a buddy who is having problems with an 87 Jeep Cherokee 4x4 Auto tranny, inline 6 cyl. This one has the Power / Comfort switch on the dash, although I don't think this has any effect on his current problem.
Anyhow, lately with the hot weather, he'll be driving along at road speed when the tranny will suddenly shift itself from OD into 3rd or maybe even 2nd gear and rev the motor in the 4K RPM range. When this happens, even if he puts the shifter in the N position, it stays stuck in the gear it self selected. Eventually it will shift back in to OD on its own.
I'm thinking it's really time for him to do the fluid / filter to start with, however, I also think there may be a servo or solenoid problem causing this.
All other the gears engage notmally, including reverse, the bands are not slipping in any gear, and the fluid level is correct with no burned look or smell.
Any ideas what might be causing this problem folks?
Thanks.