RCT 3 keeps minimizing itself

[Rush]

I have a problem with Rollercoaster Tycoon 3 it used to work fine then all of sudden it started closing and it would be minimized in the taskbar and I would have to click and start it back then it would do it again just in a few seconds any help would be apppreciated. I am going to upgrade to more memory pretty soon and wondered if it was the fact that my memory is at half right now.