Front derailleur adjustment question: what to adjust I'm a bit confused about what's going on with my front derailleur (which is PROBABLY a Shimano Acera, but the bike is 13 years old and the name has long since rubbed off).  The chain has started rubbing against it when it's in a hard gear on the 3rd (largest) chain ring.  This is the 8th gear.

If I adjust the barrel adjuster until the chain just stops rubbing, then it will invariably rub in the 5th gear.  Previously, the bike was able to work without the chain rubbing until I got into the 3rd gear.  If the barrel adjusters aren't working anymore, what might be the issue?  Is it just a cable tension issue that's too far gone for the barrels to fix?
If I were you, I would first get new cables and maybe even housing for the derailleur cables. Put the barrel adjusters in sort of the middle and change the cables. If it's still having an issue, then the problem might lie in the upper limit of the derailleur. Check the limit screws on the derailleur and loosen the upper one.