There are trade off between quality and preview performance.
NVS2500 uses special CODEC algorithm to meet reasonable speed during preview.
This CODEC has round off effect on the motion estimation of INTER frame(P and B frame) data.
So if the footage is INTRA only coded, the preview is clean.
But if the footage is LONG GOP type, then the preview of P & B frame might be looks unclear, i.e., looks like focus out. I frame is OK.
So the overall preview looks like periodic focus in(at I frame) and focus out(during P & B frame).
This happens at the preview of LONG GOP video file such as taken at SONY XDCAM.
The focus in/out is caused by preview algoritm and not the problem of your footage.
If you check it at the editing system, you can see clean footage.