2000 Ford Expidetion Speed problems?

when driving truck doesn't sift doen in gear...at 50 miles a hour the RPM is up to 3000...then it overspeeds so i hit the overdrive on and it will let me go up in speed but RPM goes up to 4000 +. Sometimes it just start working but lately it doesn't. i have auto start on my truck but i don't know its affecting it.Can it be a blown fuse or loose wiring. Someone told me it might be the torque converter might have a loose wire. What can i do?