Sounds normal. Windows is hibernating the program because obviously you don't have that much RAM and it needs to free up memory for otehr stuff. And since AI doesn't use a swap file, there is also no open file activity that would prevent Windows from behaving this way....
I have 8G of RAM and this has never been a problem until recently?
Is 8GB of RAM a lot of RAM in your opinion? You have to keep into account that you are running a 64 bit system and in doing so your applications, that are 64 bit can access a lot of RAM. So if you are runnig other applications which are 64 bit whike running Illustrator that might cause a problem. I myself have 17 GB and plan to make that 24 GB of RAM beacuse thought I can work quite fast on my MacPro it would work better with 24GB.
If you are running the Master Collection and use all the applications you might want to think about 32GBs, I know of some one who has 64GB and they say things fly on a 64 bit system when you have that amount of RAM.
Don't get angry with me fr telling you this bt it is the way of the future.