Civ 4 lag after an hour

[BiSHGoD]

I have this wierd problem with Civ 4 in that it lags after about an hour or so of playing. I have no background processes running or anything.

Wierd thing is that I've done this on 2 PCs in my house and both of them have this lag.

Both System Specs:
P4 2.4Ghz and 2.8Ghz
1GB ram on both
Geforce 5950 and ATI x800

Both run any other game flawlessly. I've reinstalled Civ4 on a seperate hard drive (different channel than the SYS hard drive). Updated VGA drivers, installed DirectX9.0c. Also I'm pretty sure both games are patched to the most current.

Is there anything else that could cause this? I'm ruling out hardware because everything else runs fine. And on one PC I have dual monitors and run Task Monitor on the second screen and nothing else is running high.

Also both PCs are ok on system temperatures, so the CPUs aren't being throttled like Pentiums do when they overheat.

This is just normal single player game play, not online or anything.

Thanks for any help.

[crunch]

What exactly do you mean by lag? Does it go away if you 1) reload a game, 2) restart game and reload, 3) reboot and reload? Do you get same lag for small maps and huge maps?