In all guns:
- Recoil spring assembly
- Magazine springs (or the magazines themselves if you can afford it)
- Trigger return springs
- Extractor spring
Check and replace only if needed (i.e. pay attention to):
- Striker (look for chips)
- Locking block
- Locking block pins
- Takedown lever
- Any other small parts
Tell anyone that has a WML on their gun to get it off.
That should bring them back to close to 100% reliability.
The root of all evil with the 40 S&W Glocks revolves around the recoil spring assembly, the magazine springs, and frame flex. Broken locking blocks and/or pins comes from a warn out recoil spring and the gun beating itself to death.
Contact your local Glock factory rep. He may just give you the parts for free (they've been known to do that). Even if he does not, he can probably help you get a quote for a screaming trade-in deal for new Gen5 guns. That typically is what they have done in the past. My guess, based upon past experience, is that it will be cheaper to trade-up then try to buy new parts for old guns.
In our modern plastic gun era, most departments are factoring in a ten year replacement cycle for their issued pistols and that is what the manufacturers are catering towards.