Do you have enough control on the host to add a stalled/hanging check and auto-restart it? Obviously that isn't a fix but maybe it'd help?
There are batch files already for this purpose, but they ONLY restart after a termination of the server. If the server hangs, it doesnt actually terminate, hence no auto restart.
I still think we should revert to basic plugins from Allied Modders (i.e. SOurcemod official ones), then add back the others over time, assessing server stability.
There really is no other way to work it out, or we're just randomly tinkering without knowing what caused the problem

@ Omni: I found it an interesting co-incidence that when the server hangs, and I go onto the dedicated server box via remote desktop, the CPU utilisation is nearly 100% accross the 4 cores. The utilisation drops after a manual restart of the server... so I guess plugins maybe triggering this action.. maybe also worth considering a race condition between plugins.. of course, resetting the server also drops TF2's memory allocation too... so we can't ignore that either.