The reason the compressor goes on and off periodically is as you're driving and the suspension moves up and down there are times that the leveling valve will dump air in order to drop the ride height back to level. The leveling valve is an important aspect of your rear suspension.
I've replaced the leveling valve on my 310 and it wasn't that difficult of a job. You just want to work safe while doing the job.
I'm not sure that I agree with the 90 psi being to high. I think I ran higher in the 310 but can't remember for sure. The leveling valve only allows the bags to reach whatever pressure is necessary to attain a level ride. That pressure could be as low as 50 psi inside the bags or possibly as high as your system will allow depending on load weight of your rig.
It sounds to me like your system is functioning as designed.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)