"Intermittently the doorphones will ring no answer "
I assume you mean when you Intercom a door phone you hear endless ringing on your end.
That's like what happens when you intercom an IP set and it's not plugged in.
What happens if you then go and press the button on that door phone being called, will it work to call or chime?
How soon after does it correct itself?
I doubt replacing 1 will help since they all have the same issue.
If you prove they are all on the same DSM then choices would be:
Loose AMP cable, not on straight & tight
Too hot in room or cabinet fan not working
Dusty MBM, not likely in a house but maybe if it was elsewhere at one time.
Power Down, reseat it
Bad MBM, swap it out
Hard Drive failing - but usually voicemail stopping is the signal
I think only station MBM's paired with trunk ports use a fan on the back of it.
I would probably just swap out the MBM since they are cheap these days.
Good luck on that one!