PMP 900 MHz Interfefence

I am seeing an issue with some PMP100 and PMP450 900MHZ radios. For the past 2 days connections fail and my registrations bounce all day and half the night. The only time the sites are stable (and there are 55+ miles between them, is between about midnight and 3am. I'm at a loss I've done SA's and can't seem to find any source of interference. Do you have any suggestions or ideas on troubleshooting this? 

While you may not see anything on a spectrum scan at any given period of time, you may still be causing yourself interference if the two systems are not properly synchronized. Have you had a look at the frame calculator tools and guides?

Both of these resources are available here.