Huge amounts of lag/crashes

This game is brilliant until ive played for around 4/5 hours straight on sandbox

I get up to alot of resources/money and then the issues start.

My cpu reached 89% and the memory was touching 90% when the lag started really bad.

When i put items into Valuts it seems to get even worse and building on here with the resources seems to crash me constantly.

Also im having problems with getting my workers to remove buildings, they just wont do it.

I really do not know why this happens, i have canceled every running program on my computer so only the game is playing, I have put all the settings to the lowest you can go, yet it still lags.

please can someone help :confused:

Thankyou for your time reading this

Computer specs-

intel core i5-3450 CPU @ 3.10GHz
8gb ram
64 bit operating system
Nvidia Gforce GTX 660