Frame rate drop with v10.3.4

Hi, I updated to v10.3.4 Standard pack and now when I play videos, I get drops in frame rate which last for a few seconds and then go away, then come back again a few seconds later and this happens throughout the video. I don't remember this happening with v10.3.3. None of my settings have changed from default (on MPC Output settings it's set to EVR custom which I think is default).

I've checked the video statistics when it's playing and MPC reports the frame rate as it should and no frames dropped. I've tried changing the mpc-hc.exe Process Priority to High but no difference either.

I'm running:
Windows 7 64-bit (Fully updated)
AMD Phenom II X4 965 Black Edition
4GB DDR3 RAM
Sapphire Radeon HD5850 1GB GDDR5

Any more info you need?

Edit: Right, I forgot to mention that I had installed Unity Web Player, uninstalled that, updated Adobe Flash Player Plugin to 12.0.0.70 and got the latest definition for MS Security Essentials, rebooted and the problem seems to be gone. I'm guessing it was to do with the Unity Web Player but can't be bothered to reinstall it to find out.

Might be fixed by the reboot.

If it ever happens again, look at MPC-HC menu > Play > Filters, to see which DirectShow filters it uses.