I tried squirting some T9 into the space where the rails go into the saddle frame, to no avail. I also tried removing the seatpost, cleaning and lubing it, and putting it back in, but unsurprisingly that didn't do a thing, either. I'm going to try loosening and re-tightening the bolts holding the saddle to the seatpost next.

BTW this is a Jett 143 we're talking about. I bought it in July 2007 and have put about 12,500 miles on it since then. If I bought another Jett, am I likely to have the same issues?