I took the bike in to the local shop for refurb, a bit over a week ago. I popped in to pick it up, and he hadn't replaced the tires. I pointed this out. He said they looked good. I reminded him I told him that they lose air very quickly, once you hit rough ground, and the sidewalls are badly cracked. This had become very apparent when I had taken it out off pavement, which is where a mountain bike properly belongs. So he actually checked them, and the condition was pretty appalling. If you let them deflate and bent them, the tire ripped right apart. And the tube actually just literally came apart when he pulled on it. Wow. BAD.
It was a slow rainy day, so I hung out and he tried tires. Now, this is a first-generation mountain bike. It was NOT easy to find a tire that fitted it easily, because the wheels/rims turned out to be a nonstandard size. I finally ended up with some expensive kevlar-banded tires that he gave me a substantial discount on, because they were the only ones that fit. He also wasn't happy with the gearing, which was slipping a bit. He adjusted it and said I should try it for a bit, and bring it back if it was acting up.
So I got home and immediately took a long ride, and worked it through some nice offroad terrain. Got the tires muddy, a few rocks stuck in the tread, you know, good stuff. :-) Gave it a good workout. Yes, the gearing has issues. It skips in the midrange, now and then, and will also shift up or down in that range, at times, without intervention. (Great, I have a mountain bike with an automatic transmission...) I'm going to work it out more in the next week or two and see if I can determine any patterns. But it was so GOOD to get out again! And it wasn't raining at the time. I was also pleasantly surprised to find that I had no issues with a nice long ride, after so long, aside from some interesting sore places the next day, since my tush isn't currently used to a bicycle seat.