I'm looking for some help creating a formula to put into conditional formatting.

I have a column full of string values and another column full of dates for those values.
I want conditional formatting to highlight all instances of an item if they occur more than 3 times in 2 months

Example:
item 1 5/1/12
item 1 1/1/13
item 1 1/2/13
item 1 1/31/13
item 2 2/2/12
item 2 3/1/12
item 3 4/4/13


So in this instance... all rows with Item 1 would highlight because there are 3 instances all occuring within one month of each other. Item 2, however, would not highlight because while their instances are close in date, there are not enough instances to trigger the formatting.

Is this possible to do with functions or would I need vba to do it?
Can anyone provide guidance?

Thank you very much!!!