Sudden shift from 4th to 2nd

This '94 Intrepid (3.3) has started randomly to exhibit sudden shift from 4th to 2nd at highway speed....Did someone report such occurrence and probable cause of such problem. Will inspect shift linkage , but suppose it can be anywhere from contacts, computer gone haywire or body valve malfunction. Any hint to shorten search?

OK....I've gone through this forum and found, sort of, that this might be a case of reverting to "limp mode" and some of possible causes were alleged. Thanks to those who have suffered before. Sooo.....will go that way and report findings when available.