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..

I have a 2000 F150 Lariat and the same problem developed for me about
6 months ago. I FINALLY got irritated enough that I tore into my door
this past weekend. I took it all apart and decided that I needed a
new actuator. I went to a local salvage yard I picked one up, only to
find that it too had the same problem. I did a lot of searching on
the internet when I finally ran across a forum that discussed this
issue. (Foregive me for I can't remember where I found it and I can't
seem to find it again). Nevertheless, one of the guys found the exact
problem and the solution. I implemented the solution and the lock
works like a charm! Basically, what you have to do is the following:
Start out by completely removing the lock actuator/striker assembly
from the door. Next, you need to disassemble the plastic black case
that houses all of the actuator parts. Do this by drilling out the
two metal rivets at the top, then pop the tops off of the plastic
mushroom head rivets - there's a total of 5 I believe. Then carefully
pry the case apart. You will see some gear mechanisms and a tiny
electric motor. Remove the electric motor. You'll notice that the
casing to the motor is primarily metal, with the back end being white
plastic. There are two metal tabs holding the plastic in. You have
to pry these tabs open (don't break them) and remove the plastic
piece. The problem lies within this plastic piece. You'll see two
brushes, and in the corner there is a capacitor. It's fairly small.
It's rectanglar shaped, about 1/4 the size as a piece of Chicklets
gum. This capacitor is the dirty culpret to the whole problem.
Evidently what they do is build up a charge, and when you hit the
button to lock/un-lock they release the charge to activate the motor -
it's like a one-shot deal. I'm assuming this is so you don't burn up
the motor by holding the lock botton down too long. These capacitors
must go bad. So the fix is to remove the capacitor with some tweezers
or needle-nosed pliers, wrap it in aluminum foil, put it back in and
it's fixed!
Obviously the tricky part is to reassemble that case you just
"destroyed". What I did was cut off the plastic bosses that used to
have the mushroom heads on them - up until I broke them off. Next, I
placed the top piece of the case back on, drilled pilot holes through
each hole, and assembled the case back together with small sheet-metal
screws. I only had one problem - on one of the screws, when I put it
back in the threads deformed the plastic and it pushed on one of the
gears. I simply took it back apart, ground off the interference
piece, put it back together, and it worked perfectly.
This saved me a bunch of time and headaches! Props to whoever figured
this out!