I don't have any experience running Apache on Windows systems, but looking at the screen snap you posted strikes me as a little odd. It looks like Apache is not using both processors. Also, the processor Apache is using is not maxing out. The load is certainly present, but it's not pegged at 100%. So the system - at least in your screen snap - doe not appear to be totally processor-bound.
When you say "Apache stops" what, *exactly*, do you mean? Does the httpd process die and need to be restarted? Or does the site continue to serve existing connections, but refuse new ones? What does the user see - a hung or lagged connection or do they receive an error (like 503)?
I see a few things, regardless:
1) Load eAccelerator or APC. That will very likely immediately drop your CPU utilization, since the opcode cache takes the load of compiling the PHP code off the processor. Frankly, I don't see reason why not to enable an opcode cache on *any* Moodle server, regardless of scale. It's all benefits with no downside.
2) Your Apache process doesn't seem to be using the other processor when it's under load. It should be able to use more than one processor. This is likely a Windows-specific config issue. Are you running Apache in worker or prefork?
3) Is MySQL running on the same server as Apache? Is there anything in the MySQL slow query log? A slow database backend can cause the Apache threads to pile up and start timing out.
Hardware and performance
Apache stops processing
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