Two theories:
1) Overextrusion and it is catching the nozzle
2) Add a fan for your X motor http://www.thingiverse.com/thing:128526
Belts could be an issue, but would be persistent with multiple prints. Personally, I slide my X carriage all the way to one end, then check tension 75% away from said end and I do this with both sides. At this point, you want it to be able to pinch the to lines together, but feel a little give....not too much force.
Belts can settle and stretch. It's just wear and tear. If this seems to be the problem after adjusting belts time after time, it would be wise to upgrade to some fishing line 
Printit Mason and Printit Horizon printers
Multiple SD2s- Bulldog XL, E3D v5/v6/Lite6, Volcano, Hobb Goblin, Titan, .9 motor, Lawsy carriages, direct Y drive, fishing line...the list goes on
Filawinder and Filastruder #1870.....worth every penny!