Moe D

I purchased a 2005 ls 450 this winter. Great car except for that dang low tire pressure warning light. Checked the spare tire for issues. No problem there. Just bought 5 new summer tires. Still have the same problem. Seems to come on only at highway speeds. 10 minutes on the Interstate and then bing, the warning light goes on. I reset it, and then 10 minutes later, on the highway, same thing. No problem in and around the city. Anybody know of a Technical Service Bulletin on this issue?