Out of interest, is the problem server a Win 2008 R2 box? I'm currently investigating various Machine Unreachable/Monitoring errors, and have found a rough correlation with Out of Memory exceptions in the Base Monitor log. These exceptions may be thrown by a misbehaving WMI provider service on the monitored box, in which a memory leak causes the service to increase in memory usage until it reaches a 512MB cap. The service will eventually recycle, but will effectively disable remote monitoring until it does.
I'm not saying that this is definitely my problem, or yours, but it is a potential line to check, particularly if (as in my case) you have multiple monitoring systems which both use WMI.
For ref: http://support.microsoft.com/kb/981314