Such issues are very difficult to tackle because there are so many people involved.
Since you and your providers say you have tried everything,
I will point towards a problem inside the FR/ATM core network.
This could be a problem on the ATM side not well communicated towards the FR side.
FR/ATM interworking is not perfect.
I suppose you do not have much control on the ATM side,
since you post only information about FR endpoints.
Your PVC consists of many little pieces that could switch between different paths
inside the FR/ATM network for various reasons (physical circuit or cabling problems,
card failures, improper clocking settings, and who knows what else).
This rerouting is done to protect PVCs from long outages, and if it occurs,
it is a good sign that your provider is struggling for availability.
Such reroutings are usually massive, so they can't go unnoticed.
Because many circuits are rerouted, this can take time and result
in short outages. If your PVC cannot be rerouted
because of lack of available bandwidth, you will experience longer outage.
When the failure condition is cleared, PVCs could be switched back to their
original path (either manually, automatically, or scheduled) and you might
see another short interruption in your service.
Ask your provider's customer support if they see failures of your PVCs
or frequent reroutings of your PVCs inside their network core.
(Supply them with some details about the time that this occurs,
so they can search their log files.) If they cannot answer you this,
they should escalate the problem to the people who can.
If your PVCs are indeed frequently rerouted or failed, you can ask them to
temporarily force your PVCs through the most reliable path (if one exists),
until they resolve the root cause of the rerouting or failure to reroute events
(either inside their FR/ATM equipment, or together with their physical circuit provider).
Remember afterwards to tell them to let your PVCs free to twist between paths
for the future case of failures in the currently reliable path.
They could have many PVCs and forget the gave you some specialized treatment.
I sympathize with your problem (and the providers ;-).
Good luck!
