RE: XPS 15 fan, on/off/on/off, gahhh!!!
I've switched to a mac 2 years ago, but the XPS is still in use by someone else.
I can tell you this: I've got the Core i5. When I bought the laptop, the fan would switch on/off very frequently according to CPU spikes instead of actual temperature readings. That issue was kind of fixed with a bios update.
The laptop is always warm and gets hot very fast. It doesn't really matter what OS or how fresh it is (tested W7 and W8). Even without doing intensive tasks, such as browsing the internet with Outlook open, the fan is 100% most of the time. I didn't try anything on a hardware level (thermal paste, downclocking) to solve the problem because the only way I got the laptop to stay relatively cool, was with throttling the proccessor with Power Options with maximum processor state to something like 50%.
So my best guess is bad internal architecture which leads to overheating no matter what.