If you inflate your tires to 120 and then ride for a while, what's the pressure at the end of the ride? I think it wouldn't be unusual for a high-pressure tire to lose 5# or even 10# over a 6-8 hour ride. I've never put a gauge on mine at the end of a ride, but they lose 20-25# from one day to the next.

If they're losing more than that, it's most likely a slow leak or possibly a bad valve, but it would be unusual to have the same thing happening with both tires at the same time.