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!