For the last several months, my 1992 LS 400 seems to start out in a lower gear, take too long to downshift and then decelerates like it was in a lower gear when I take my foot off the gas. It takes more RPM to get the thing moving to normal street speed and is drinking gas like mad. Problem is getting worse. Repair guy says the transmission seems OK, but he can't figure it out after 5 days. Clues?