STP Loopguard - Blocking the root port?

Hey all. I've got a (hopefully stupid) question.

As I understand it, loopguard will transition a port to loop-inconsistent state if it receivesstops receiving a BPDU. On our campus network, we use MST for the vast majority of our switches. However, we have some switches where we cannot run MST (they support it, but .... reasons). We also must enable loopguard, as per our security guidelines.

On the switches where we cannot run MST, we are running PVST. On those PVST switches, sometimes (not consistent), the root port will transition into a loop inconsistent state. The topology is very simple - the PVST switch is connected to an MST switch via a single cable (sometimes it's a fiber link, sometimes it's copper). There are no redundant links involved in that portion of the topology. Disabling loopguard will resolve the problem, but it can reoccur if loopguard is re-enabled.

So, my questions are:

1. Why would the switch transition its root port to loop-inconsistent? Because it stopped receiving BPDUs, right? Leading to #2....
2. Why would an MST switch stop sending BPDUs to a PVST switch?
3. Why is this a very intermittent issue? It doesn't happen on all the PVST switches, it can go days, weeks, months without the issue. But once the switch has the issue - it keeps happening.

Note: This morning, I changed some of our PVST switches to Rapid PVST - I hope that will resolve this issue, but only time will tell.

Thanks for any help you can provide!