I have a stock 01 LX470. Tires are in good shape on stock rims. Whenever I drive for a prolonged period of time above 60 -70 mph, I lose air pressure in one of my rear tires. I've had the tire removed, dunked and checked for leaks a couple of times, with no signs of any leaks. I've read this can happen to tires with a TPMS, but I don't have those. In those cases, the solution has been to tighten or reinstall the valve stem. Anyone had this happen?