Speedometer problem in 96 Caravan

Please help!!! My speedometer started acting up about 2 weeks ago. While I was driving along at about 45mph I looked down and the speedo said I was going 0 mph. I've replaced the speed control sensor and the instrument cluster. I still have the same problem. Just for a twist it starts out just fine, it isn't until the engine is warmed up that the speedo goes out. I did a check and showed a code 15 and a code 37. Could the problem be the input and output sensors? If so where are they and how hard are they to replace? If that's not the problem could you lead me in the right direction? I'm about to go bald from pulling my hair out (or from all the stuff I had to use to get all the transmission fluid out of my hair! LOL) Thanks for any help.

Code 15 is the speed control sensor. It's behind the oil dipstick or just to the right of the dipstick. Code 37 is the torque converter clutch solenoid(what ever that is) we still can't figure 37 out.