i have a 2000 ford lightning. had no problems with it but i notice sometimes when i slow to stop it gets rough idle and almost does dead. i notice if i down shift manually into 2nd then 1st while slowing to a stop it doesnt have any problems. the other day when it started acting up i downshifted it but once back in D and going down the road i realized it would shift up to 2nd but wouldnt go into 3rd. at the top of 3rd at high rpms its as if giving it gas did nothing. slow down a bit and it would continue to drive in 2nd as long as the rpms werent too high. parked it then about 3 hours later it shifted fine until the next day it started not downshifting again. after about going dead at a stop sign i manually downshifted again and now its back to not shifting up out of second.

 

I would get the OBD2 scanned with a scanner that can read Ford transmission codes. If there are any codes, research them.