So just summing up from what you said long periods of inactivity seem to be when this happens.
When suffering MSN downtime does your Plus! events always record the information? From past experience my event log hasnt always recorded when ive been d/c'd from either my network/router or even MSN as I have automatic login enabled. So it passes without me noticing it. This hasnt always been recorded in the event log for me, I cannot say this for anyone else.
It would be interesting to find out if this is the case in your situation, as its a random uncommon problem happening during long periods of inactivity.
Sorry I can't add any further advice, but maybe a bit of testing to see if all 'log-outs' are actually recorded?
Johnny.