I'm not sure if your problem is the same, but it sounds a lot like the problem I had with my 1995 LS400. It turns out that there was a service bulletin on this. It was very easy to fix and only took about 15 minutes. Basically, all I had to do was unravel some tape on the sensor position harness. If I did this right, the link below should take you to the TSIB.
http://www.clublexus.com/index.php/article...view/2077/1/41/