Transmission failure to upshift at highway speeds?

Recently my fully serviced, fluid full and working very well, 3.0 3vze 3.0 A 340F Automatic Transmission began failure to upshift at highway speeds. The RPMs are at about 4,000, at 60 MPH. The Throttle Kickdown Cable outer cover is somewhat worn, but the inner cable moves freely. Also the Electric Over Drive Button suddenly has no effect during these episodes. Sometimes I can stop the engine, re-start it and it operates properly temporarily. I am perplexed? I ordered and plan on installing or having a dealership install a new Kickdown Cable to start with. Is there any other components that may cause this? Where to begin troubleshooting? At this point, I have so much money in this old vehicle, I can only keep adding new parts until this final and sudden issue arose. Any help from a mechanic, very experienced and familiar with the Second Gen 4Runner Automatic Transmission would be more than greatly appreciated. Thanks.