My wife's 2001 RX 300 VSC and CEL come on after driving for about 3 hours. After stopping for fuel, they both come on immediately after the engine is started. This has happened 3 times after driving for about 3 hours each time. After resetting the codes, the lights stay off, until the next 3 hour trip. Although I can't be sure, I believe the code number pulled was P0172 - "lean condition, bank 2". I'm sure about the description though.

Any thoughts on why this would occur? Gas cap, maybe?

Thanks!