This is probably a graphics issue.
There are a number of steps you can take to make things easier on your graphics card, and many of those steps have been explained very clearly
HEREAlso, you may want to look at how many trees and mirrors you have on your lots, as these are both known to require a lot of graphics commitment.
Your best bet is likely to be upgrading your graphics card. Depending on your system and resources, I would recommend anything in the ATI Radeon 9 Series, and preferably with at least 128MB onboard, though 256MB is preferential.
If you have any questions about what I've suggested, feel free to send me a private message.
Good luck.