My 1991 Honda Accord EXR seems to have a problem with the torque converter not locking up. You can tell because at 100k the RPM is at about 3000rpm and once the car gets warmed up (about 1/2 hour of driving) the RPM drops to about 2600rpm and you can tell the converter is locked in. Any ideas of what it could be.

Access whether changing the ATF and filter might help. This should really be done every 30,000 miles. If this has not been done regularly, it could make a world of difference in how it shifts and performs. If you think the transmission is basically in pretty good shape, I'd use Amsoil Synthetic Universal ATF. Synthetics run much cooler and keep the transmission much cleaner, which is very important for today's electro-mechanical transmissions. They are more involved than engines and demand pampering with the best fluids and service. This is particularly true of Japanese ATXs.