Kernel panics are almost always hardware related. Once in awhile they come from poorly written software, but you were right to replace the RAM.
Apple is also pretty good about helping you diagnose these problems. If you are convenient to an Apple Store, make an appointment with a genius and they'll put it through its paces to see what's causing the problem.
Another tool I'd strongly suggest for your arsenal is Tech Tools. A copy comes with AppleCare or you can buy the Pro version. (If you already have AppleCare but lost the CD, you can download it here by entering the serial number of your machine: https://support.apple.com/techtooldeluxe/main?id=dl)