Help please! I have a 1986 Fender Stratocaster that I've been playing for 29 years, and just recently one of the fine tuning screws on the tremolo has fallen off. When I try to screw it back in, it just pops out again ( as if the screw has been worn down ). Anyone know how I can fix this? Any suggestions would be greatly appreciated...

BobbyCrispy :-)
Here are a couple of things to try:
1) Have you tried a new tuning screw?
2) Look at the threads on the fine tuning screw that keeps coming out. If it is really worn, as you suspect, you MIGHT be able to SLIGHTLY tighten the fit by carefully squeezing the thread area on the screw in a pair of pliers or maybe a vise. I have done this on other hardware, though not necessarily a guitar's bridge.
3) IF you can get one, replace the part that holds the screw: be sure to get a new screw. Look online for guitar parts if Fender can't supply that. I have never tried this on a Fender, but I did do this on several Floyd Rose types.
4) If none of these work, about the only thing you can do is buy a new bridge tremolo assembly and either install it in place of the original, or swap out parts.