mav's right about the centrifugal force making yuur valves pop out a bit... you valve caps are a secondary seal to the rubber seals inside the valve which is held down by a spring, if the spring is old and weak and the caps aren't sealing properly then the air could easily leak past.
First of all you should be checking the pressures with the same pressure guage all the time (no 2 are callibrated the same), then it might be worth spending a few pennies on some new valves... not the whole valve, but just the springy bit inside which you can get from halfords.
If you've got porous rims then that's a lot of bad luck and a lot of money to replace. 99% sure that it's either the valve internals leaking, or you're not checking the pressures correctly. Try doing the pressures clinically, i.e. at the same time of day at the same abient air temp, with the same guage, after they bike's been sitting overnight... and if that still shows a significant drop then replace the valve seals.
p.s. take no notice of this post because i'm a little under the influence and don't even know how i managed to turn the pooter on, let alone think about answering your question! T'was a good kareoke night tho
n