Do you notice that the problem is heat-related? In my '95, whenever the trans fluid got hot, the TCC would not lock-up. In my case, adding a transmission fluid cooler mostly solved the problem. I installed the $40 model from http://www.bulkpart.com
and the trans now stays cool enough to shift and lockup normally on all but the hottest days. If I had bought the $60 model, maybe it would work fine even then.
Anyway, even if this does not solve your problem, it will not be money wasted. Keeping your trans fluid cool is probably the best thing you can do to prolong the life of the trans. An added benefit is that the engine runs cooler, too.