I was experiencing very similar issues to you on the exact same systems, I live a few miles north of you. We are probably receiving from the same towers. I read a variety of posts on RadioReference and there appears to be a lot of issues reported on P25 simulcast systems, of which I believe Delaware County P25 is a simulcast system. Here's what I have done which seems to be working from a perspective of being stationary at home with either an external antenna or OEM antenna (made changes Friday evening, so I only have several hours of monitoring with the new changes).
1. I created a new file from scratch (I use the ARC500 software)
2. I changed the default setting on MULTISITE, was 'ROAM', is 'STATIONARY'
3. I changed the default setting on CHECK ALL CC, was 'NO', is 'YES'
#2 and #3 were changed both for DelCo P25 and MARCS trunk systems.
The URL below is to a post that clearly articulates the Multisite setting as well as Check All CC.
Do the 106/197 and 500/600 do multi-site?
I have not tried setting Multisite to OFF. If I experience any more issues, I will try this next. All other settings are at default. The attenuators are not activated.
I would be interested in hearing your results if you try #2 and #3 above. If you have any questions, post or send a PM.