Good to see you around again Tango.
Yeah, Rob did mention in chat that it could be a driver issue as well, but the method he posted is just a quick and dirty fix to get rid of it if you can't afford to reboot the computer, like in the event it's 5 hours in to a 12 hour encode, etc.
The keyboard issue you mention will most likely be the result of a USB limitation. Under USB, keyboards have a max key press count of 6 - no more than 6 keys may be pressed at the same time and sent under the same poll request. PS/2 didn't have this limitation as it was interrupt driven. (To be honest, MOST people will never notice or even type fast enough for this to be a problem, though some may notice lag - called ghosting, the delay between a key press and it appearing on screen.)
However, there is also a matter of grids; most low end keyboards can only recognise 4-6 key presses in localised areas, if you try and press multiple keys (or different keys in quick succession) that are in different grids, then you'll end up with only 1-2 registered key presses.
There could also be a third problem.... dirt and just wear and tear. Self explanatory really, dust gets in under the contacts or the contacts themselves get warn out.
The last problem could be a mix of either Windows has a bad HID driver or you have a bad USB port. Swap USB ports to a different pair (not the second port on the same pair). This will usually reinitialise the HID driver at the same time.
If you still have issues, the short of it is likely to be, get a new keyboard. If you do a lot of typing, then investing into a good mechanical keyboard will likely ease a lot of problems - if you can get past the feel of them. You can get silent ones, but you'd have to look around - plus, they ain't cheap. Otherwise, go with high poll rate typist or gaming keyboards.