You are one of the few who check your tires on a regular basis and they are all fine except on one vehicle that is front wheel drive with aluminum rims. You have had it checked several times to no avail. Your problem is corrosion between the tire and the rim and it is worse because the front wheel drive causes the tire bead and rim to flex. The cure is to take it to a tire shop and have the rim bead sealed. I have a 94 Pontiac Bonneville, my second 94 Bonny, with aluminum rims and this had to be done. The other solution is to inflate with nitrogen it is very dry and has no moisture also the molecules are larger so the leak is less. The other thing you may have to do when you install nitrogen is to have the rim cleaned so you may just as well have the rim bead sealed and save the hassle of nitrogen. Nitrogen seems to have dropped out of the limelight lately I thought it would take off if the Manufactures installed it originally, but they are having trouble affording air right now.