The other day I was driving down the highway and out of nowhere my '99 downshifts. I had the cruise control on, but there wasn't any reason for it to downshift. It was like it clutched down, except I have an automatic. Unfortunately I can't get any codes from it if it has any. I haven't changed the transmission fluid in a bit, but I've never had as issue with fluid level or anything. I was thinking maybe the kickdown cable?

 

If you have a 4.7 engine I'd look at changing the TPS, they get worn and sloppy. No CEL/DTC's as the computer thinks your moving the accelerator/gas pedal.

 

I have the 3.9....