2000 Lincoln Navigator

I need help determining a problem with my navigator. The engine light came on first, then the overdrive light came on and continued to flash. When the truck gets to second gear it jerks violently. I had a mechanic put it on a machine, and was told that in needed a new transmission solenoid. I had that replaced for $650.00 but when I drove the truck again the same two loghts came back on, and the navigator started shifting violent again. I took it back to the shop, and so far they can't determine what went wrong. Has anyone else experienced this problem? If anyone has any information that can assist me it would be greatly appreciated.