Do not use WD-40 or spray Lithium Grease, they'll gum up the actuator assembly or muck up the minor amount of electronics in the actuator assembly (I know that seems counter intuitive since there is grease in the assembly, but we have a decades long tech-tip standing from Toyota about using cleaners/lubricants around the actuators and premature failure). If you want to use something, try a fast drying silicone lubricant, it's not ideal but at least it'll limit the amount of gumming up in there.

The next time your door starts acting up, open the door and bang on the latch end of the door (either with your fist or a deadblow) IF the actuator starts working as if nothing is wrong and/or still acts weak, it's failing and needs to be replaced.

There's two routes you can go; the first is getting a factory actuator assembly and replacing it yourself (or having a shop do it), the second is finding the part number of the electric motor inside the actuator assembly (Dealership can't help you here, I believe some old threads might have that information, usually you can order them online or get them from a hobby store) and rebuild the actuator yourself (if you do rebuild it, get a little bit of white lithium grease to relube where the factory lubed the original actuator when you have it apart.)

I would not recommend an aftermarket actuator unless it's made by the Denso or Asin (the two OEMs who make our door lock actuators usually if I remember), I've seen a lot of crib deaths and early deaths with those auto-parts store units and honestly even how much cheaper they are, I'd hate to replace the damn things yearly/monthly.

Hope that helps.