I had the same problem with trans shifting into o/d and after about 10 seconds the converter would come out of lockup and trans would downshift into drive. After two trans
rebuilds(complete),new pcm, valve body,converter,gearset,sensors,ect., The front pump which always tested ok, blew out. After replacing the pump, the trans has worked perfectly. The trans mechanic was amazed. I just wanted to give my situation and fix because I put up with this problem for 2 frustrating years. hopefully,this might help someone out.

I'm experiencing this same problem...does anyone have any other ideas??? Also how tuff is it to change the pump in the trans if it is the problem ??
Thanks John

It would be nice to know your year and trim level. Can you put that in your signature?

I have a 94 ssei bonneville that seems to shift into overdrive and converter lockup at about 55 mph (not getting on it just gradually increasing) then acts like it'* too high gear for the speed, boost gage goes to about 8 psi and feels like it'* really loaded. Then about 65 mph it will downshift automatically into drive and stay there. Even cruising down the highway at 70 or 80 it stays in drive about 3k rpm. I know it'* in drive cause I shift to drive manually at 80 and nothing happens. When it does shift into overdrive and TC locks up between 55 and 65 if I tap the brake I can feel the TC unlock then I can downshift to drive and it will downshift.

Your problem sounds quite different. I suspect too much TC slip in lockup with your car. You better check the condition of your fluid, not just the amount. If I'm right, a good professional flush might buy you time or fix it outright.

The other possibility is a bad TCC solonoid unable to regulate TC duty cycle.

I know this trans was a replacement. I don't know if it was rebuilt or just a junkyard swap though. I also got code 740 so I probably should just go ahead with fluid, filter and tcc solenoid and see what happens.