Gigablue HD Quad HDD problem

Hey guys, hoping someone can help here, have gbquad, running HDFreaks recent image, changed image couple weeks ago could be problem? My issue is when watching programmes as an example just paused dragons den then watching on time shift am getting slight pixelation 'slight freezes' noticed this over last week had thought it was down to recording something else and reading from HDD at same time, but not using HDD for anything else at moment, any ideas would be appreciated, had looked to see if there was disk defragmenter tool or similar? Didn't mention its a 1tb HDD probably more than half full been busy downloading via 1channel and off usual route just one of those annoying problems