IMac Lagging

I have noticed over the last few days my iMac has a lag to it.....even as I type this about every 10th word is sorta pausing for 3 seconds....I am not running any other programs. On occasion the beach ball comes up....but not all the time....just a general lag.

I bought it new about 9 months ago and haven't really installed ton of new stuff on it. I have lots of memory available.

I am not particularly computer savvy, but can anyone tell me where to start with my problem. It becomes a real annoyance at times.