This probably wont be much of a help but I thought I would respond with the end result of my original post above. The intermittent problem you are explaining is pretty identical to the one we had, except that its not the same set or rears going flat everytime. On ours...my husband took apart all the connectors, no shorts in wires or bad connections. Pulled the computer and it looked brand new. Eventually when he put it back together for the second time, the whole thing stopped working, the compressor wasnt going anymore either. So rather than rack his brain any longer at fixing it because it wasnt something obvious, since all the fuses and wires were fine, computer looked new (could have been busted but would require someone familiar with testing it, or getting a new one), compressor worked fine, air bags will fill and hold fine when told to, etc...He just went to PAP and got regular strut system off of a GL wagon in the yard and slapped that on, and its been perfect ever since. The only thing he thought possibly was not working right were the silinoids, but we didnt want to put that much money into the car to get new ones since no junk yard around here had any.
From everything I have read about others having trouble with this system is the end result is taking out the air system and putting in a regular one. They seem to always eventually have problems and pricing replacements was an eye opening experience
But if you have the time, patience and money...I hope you are able figure out what the problem is.