The compressor is easily serviceable and the vent solenoid can be resurrected. Its a project, but Jamie is right in that the vent solenoid is probably sticking open at random times.
When the car wants to raise the vent solenoid is closed and solenoids on the bags open. First front, then rear is leveled. If the vent solenoid sticks, the bag solenoids still open but the compressed air just escapes right away and the bags stay open since they expect air. This can cause a slam.
The compressor comes apart very easily. I have serviced two units. There is a single piston which I lightly lubricate. The vent solenoid is sealed, but I soaked a sticking unit in a blend of C.L.R. and water (calcium, lime, rust) solvent and then alcohol to displace the solution and water. Once dry I lubricated with silicone spray lubricant. Going on a year now and the owner says its still working fine although I dont think it will last as long as a new replacement.
Another option is to somehow find a Spider valve, but since they are no longer made they are hard to find. But it replaces the stock vent solenoid valve for a new one.