Automatic transmission won't shift down to first gear.?

I own a 1995 Acura Integra LS and I've been having some problems with the automatic transmission. Every now and then when I come to a stop, the car will jolt and make a knocking noise and fail to shift into 1st gear. It will run normally after this except for the fact that it starts out in second gear every time I start out. It will continue to run like this until I manually shift it into first gear, then it will return to normal for awhile. Even though this isn't a major inconvenience, I'm worried that it's a sign that the transmission is starting to go. Does anybody know what might be causing this? Is it a solenoid problem? Low fluid? Is it a major problem? I'm going to be selling the car in the near future and would rather not sell a car with a problematic transmission...