I, too, have had OD problems recently (arrow turns on and off, solenoid makes proper clicking noise, but intermittent drop from 4th to 3rd gear.) I looked around for all the common suspects but found nothing obvious. I am not familiar with checking for electrical problems so I did the best I could without using a meter of any kind. In the end I did 2 things and have not had any trouble since:flushed the transmission and did the Dremel modification to the solenoid. I was comfortable doing this because I figured it might save me much more OD grief in the future and because I don't recall ever having used the OD button to lockout 4th gear and don't anticipate needing it in the future. My transmission is now a nicely functioning full-time 4-gear tranny that gives me no trouble. Neither the flush nor the solenoid modification were very involved or time comsuming. By the way, my mechanic had recommended a ($1400) transmission rebuild. My total cost came to less than $50. Good luck.
-Chris

PS- here is a link to the IPD flush instructions they provide with their special hose. http://www.ipdusa.com/pdf/PI-241attrannyflush.pdf (but you don't need their hose to do the flush)