Dell Latitude E6420 is lagging after BIOS update to A15

Greetings!

My machine is: Core i7 2720 QM, 4GB RAM DDR3 1333, Intel QM67 Express Chipset. Bundled HDD (500GB) I've moved to the media bay instead of DVD-RW, and on it's place plugged SSD (OCZ Vertex 3 240GB). No other modifications were done. Service tag: HXKH3R1

OS: Win Server 2008 R2 SP1

I've decided to update BIOS from A06 (original version) version to A15 (updater - E6420A15.exe), because there were some fixes I needed. But update to A08 (E6420A08.exe) is required in order to update to A15, so I've updated my BIOS to A08 first, then to A15. Each update was successful.

And then suddenly I've noticed, that my system started lagging for a very short moment (~0.5 - 0.3 sec) in random periods of time (2 to 10 mins). Lagging means, that at this very short moment everything is freezing, video, audio, mouse pointer. Now It's terrible to listen to music or to watch videos constantly lagging laptop.

Reinstalling Windows not helps. Applying default configuration in BIOS not helps too.

I've tried to downgrade to every previous version of the BIOS : A14, A13, A12, A08. A07. A06, A05, A04, A03, A02 and event A01, but still no luck.

What's wrong with new BIOS? How can I get rid of those annoying lagging?