Version of Windows? If new thermal paste solved the problem last time, maybe it needs doing again? You can try using canned air to blow out the dust bunnies on the motherboard, heat sink, fans, video card, case vents etc. Does something start running in the background you don't know about that's causing things to heat up, like HDD disk defragmenting, antiviral scan, Windows indexing the HDD, etc.?  Does this happen if you boot in Safe Mode and use PC for a while, when those things won't run?  
I have a dell XPS a2420 and most of the time the fan speed stays quite, but randomly the fan will start to speed up when doing the same thing I was doing when the speed was low. New thermal paste was installed about a year ago.
Dell XPS a2420 Fan Runs on high