Septic aerator pump alarm

Need a little help diagnosing my aerator alarm. I had my tank pumped earlier in the year. A few months afterwards I got an alarm on my aerator, checked tank and noticed no bubbling. I rebuilt my pump (noticed my diaphragms looked fine at the time, but replaced anyhow) and it worked fine for a couple of days, then alarm again. I then replaced my diffuser and system operated properly for a few days, then alarm again. I had a different pump lying around from another project so I replaced pump altogether and it works fine for a day or so then alarm again. I can unplug the pump for a couple of hours, plug back in, and it will run fine for about a day before I get an alarm. I'm wondering if there is a break in my pvc line between my pump and the tank, but that doesn't explain why the system runs fine for a day before alarming. Another thought is the diffuser is getting clogged and may have to replace or make one, but then again why would it work when I leave it unplugged for a couple of hours then plug it back in? I'm getting the same problem with 2 different pumps, so the problem isn't the pump itself.