Interesting problem on the only triple chainring bike I've ever owned (I've never run into this problem on a double chainring ever). It has a 9-speed cassette. In the middle chainring, the middle 3 rear gears hunt around. Example, when in 5th sometimes it'll randomly jump up or down a gear. 6th, sometimes it'll jump down 1. 4th, sometimes it'll jump up 1. It'll stay in the gear you put it for the majority of riding, but occasionally it'll suddenly just hunt around a little within that specific subset of gears and then settle down again.
It's been like that since brand new (2006, Cannondale with Shimano components, most notably the Flightdeck shifters). My wife just lived with it (it was her bike originally). When she crashed it (and gave up cycling), I inherited it and I also just lived with it for awhile. Finally the last couple of years I've re-calibrated the shifting a few times in an attempt to solve this issue. I can get everything spot on except for that pesky hunting right in the middle gears!
Tips/tricks specifically for adjusting shifting on these triple chainring bikes?