Keystroke lag and general hesitation even after clean re-install

I have searched the web and not found any solutions to this. My fairly new Win7-64 Dell XPS laptop is now suffering from slow downs/hesitations. Benchmarks, however, seem as good as before. The biggest problem is with typing. Keystrokes are not recorded immediately and can lag many characters behind....up to 20-30 or more if I type really fast, although they all eventually show up. This happens in Firefox, IE9, and Word-2010. Notepad, Wordpad, and Thunderbird are fine. This was so bad I started over and did a clean re-install of Win7 but it is still doing it just a day later. Also, videos are very choppy and missing many frames. Everything seems to have some sort of hesitation.

I have found lots of suggestions on the web and even some on this site; but no solution yet. There must be some clue as to why the problem exists in browsers, but not Notepad. I disabled everything in msconfig but it still does it. I disconnected all LAN/Wireless; still does it. I tried safe mode, and while it appeared a bit better, it was still there. Some have suggested it was the charger/battery; nope. Ran CCleaner; and while it seems to help Firefox for a few minutes, it does not last long.
This is driving me crazy. Does anyone know what this could be?

If it happens in safe mode, a good bet is that it is a hardware problem
Most likely hardware/memory. Take this memory test and tell us how it goes.
http://www.geekstogo.com/forum/topic/246994-guide-to-using-memtest86/
also

Test Hard Drive
Hard drive scan usings SeaTools - SeaTools for Windows | Seagate - Both long and short tests

Well, just to answer my own question; the problem turned out to be the Hard Drive!. A second clean install did not help. Swapping memory did not help. Finally, I cloned the C partition of my HDD to another drive (an 80GB SSD.) Problem solved! Dell is going to send me a new HDD since my laptop is still under warranty. I currently have a 650GB Toshiba HDD.

I guess the HDD is having some issues with rapid writing, although it seem to otherwise behave OK. The SMART data also looks OK. I first noticed this behavior about 3 months ago; and it seems to be getting worse. Maybe it is sending out some sort of interrupt to the CPU???? I would love to keep the SSD installed, but I need more capacity. I will post back if the new mechanical drive does NOT fix this issue. It should arrive in a couple of days.

There could be boot sector corruption of the HD needing wiping with Diskpart Clean Command.

There could also be a bad connection, cable or port.

Did the HD extended CD test find anything to repair? For Toshiba run Hitachi Drive Fitness Test

Thanks. I have not tried those tools yet, but I may when I get home tonight.

One other tidbit: Last night, I tried watching some of the Super Bowl commercials that were posted online. As before, videos kept pausing every few seconds. I then started a simultaneous defrag of the C drive. As long as that was running, everything worked perfectly. As soon as the defrag was complete, the problem returned. It acts like the HDD is going to "sleep" if not constantly being pinged. (I know, not a very technical explanation!)

Update: While I wait for the Dell drive, I temporarily replaced my HDD with another mechanical drive (although slower). The problem is definitely gone.