O/D Light Flashing???

my ranger is doing the exact same thing. it happened when i swapped in a 01 tranny into my 95. i usually just cycle the key off and on realle quick and it resets. it only flashes at certain times. always seems to be at the same point of my drive home. i thought there was something funny about that
 
go to autozone or somewhere that has a scan tool. Pull the codes and that will give you some idea what you are looking at....I had the same problem on my 02 GT auto, codes indicated that there was not a drop in rpm's from 2nd to 3rd....I flushed the tranny, changed the filter, filled, and reset computer....no o/d light since. Also, has he done a tune and changed the tranny shift pressure? If so, he might want to put shift pressure back to factory and see if light comes back. Sometimes it turns out you have a shift solenoid going bad.