I have the exact same problem here. In the morning driving to work I have to go on the highway right away and it works fine. On the way home I have to go through the city then onto the highway. It is at this point that OD doesn't work. I haven't gotten around to doing any serious troubleshooting yet. It is nice (not really) to know that other people are having this same problem. I did see a TSB about this and it was the wiring harness within the transmission, a diode that shorts the two wires going to the TCC because the ends weren't cut down and it eventually rubs and expands to short the TCC solenoid.
I should just get off my ^%$ and get the link here...
I don't believe that the transmission will shift into OD without the TCC being locked up and I am fairly certain that the TCC is not locked up when I am having the problem of no OD.
1992 Bonneville SSEi
White, grey Leather interior.
Tinted windows and blackouts on headlights.
Custom high flow exhaust, Everything replaceable - DONE
300 000 kms original engine, tranny 2 rebuilds.
Playboy Bunnies Embossed in Tint. Oh Yah!
Ever Seen a Polar Bear in a Snow Storm?