The wireless remote unlock in the 90 LS I used to have worked intermittently for YEARS. Shortly after I sold the car in 2003, its new owner found that the ROM chip in the ECU in the trunk on the driver side was not firmly seated. After he snapped the ROM chip in place, the the keyless entry system worked fine.
I'm guessing the ROM chip was dislodged when I hit a bump several years after the chip was replaced. The ROM chip and the keys were replaced under the original warranty shortly after I bought the car due building security systems interfering with the wireless unlock system of the LS. I'm guessing that the replacement ROM chip had not been properly installed.
Your problem could be something different but check the ROM chip. The following thread on another forum describes how "Toby", who bought and is still driving my old 90, reseated the ROM chip: http://us.lexusownersclub.com/forums...=1&#entry79653