I have a 2000 F150, and I have replaced the door lock actuators THREE TIMES !... they work for a while, and then get weaker and weaker..
I've checked the lock mechanism for binding, etc and it's all working silky smooth.
I finally just junked the lock mounted OEM style unit, and used an aftermarket push / pull unit that attatches to the rods in the door. Works great, and is C H E A P er than OEM !
But have others had problems with these things ? I've got other cars that are 15-20 yrs old, and the door locks still work great..