It may be a filament issue, or your extruder has a problem. If the diameter of the filament varies significantly, it can either jam the extruder because it's too fat, or produce thin spots where it is too thin. I have a MakerGear M2E, and there is a tension adjust on the extruder drive system. Mine was initially too loose, and occasionally it would slip, producing holes in my prints. I've also heard of problems with "lumps" in some filament that don't melt uniformly, or can clog a fine nozzle.
Also, if you try to print too fast, the extruder may not be able to melt the filament fast enough for the drive system to be able to push the filament through. Given that the problem is intermittent, you may be operate "on the edge", and some minor variation in the filament is pushing it over the limit.