I'd say either the CPU or the RAM.
That CPU is quite old architecture.
The RAM is very low, which would lead me to believe that it runs out half way through a campaign.
Well, my motherboard can support up to 16GB, but it's DDR2. Is DDR3 worth the upgrade?
It's not an overheating issue, my video card and CPU stay (relatively) cool at 38°C & 47°C, and 46°C & 50°C through a horde, so it's likely the RAM.
I realized the processor is old architecture, yes, but I got it as a gift, so I can't really complain.