Years ago, I survived my "intro to hardcore xc" with a GT Backwoods and 24-speed Alivio. This included the infamous Slickrock Trail, Porcupine Rim, Behind the Rocks, and Gemini Bridges trails in Moab, UT. I rarely had any issues with "skipping" gears or missed shifts. When I did, it generally corresponded to shifting under load. The LX setup I upgraded to a year later "skipped" gears and missed shifts almost as often as the Alivio did (once or twice per epic ride) but it did shift much better under load. My current XT hardtail does better but my latest LX full suspension does not.

My experience from tuning and maintaining the old steed would be to check if:

1. The chain is an OEM piece of dog doo-doo that has some sticky links.
2. The derailleur hanger is ever so slightly bent giving you five good cogs and three bad ones.
3. The mechanics at your LBS are complete morons that should be removed from the gene pool.

I ended up spending $50 on a bike maintenance class and started fixing my own rig after the LBS mechanics missed the first two items as well as some bad rim tape on the road bike. I have since found a better LBS but still prefer to do most of my own work. BTW, I also had a Bianchi hybrid where the shifter cables continued to stretch for an entire summer since the weren't sufficiently "pre-stretched" prior to installation. New inner wires can work wonders and only cost $5-$10 if you can figure out how to adjust the cable tension yourself.

See http://www.parktool.com for a few tips on how to check for these things. Be careful though. Getting adventurous can get you into more trouble than you're in.