Windows 10 PS/2-keyboard stuttering video/sound


maybe you know me from my xp hibernation wake-up problem.

Meanwhile I'm trying my luck with windows 10, because I've bought a new graphics 5 days ago card for which amd is no longer developing xp drivers (it's an R9 390, one of the first completely without xp drivers).
To be honest, I now have to make a decision between keeping this card and continuing with win10 or sending it back and replacing it with the old one again or getting a R9 290X, which I hope still got xp drivers.

My problem is as follows:
I have an old and cheap but very well made ps/2 keyboard I'm using at my pc.
And one of my main tasks at my pc is using it to watch tv using mytheatre.
As soon as I press any key on that keyboard (especially when I stay on that key and don't release it), any video application I've tested stutters.
The tested applications are:
- MyTheatre
- Windows Media Player
- youtube @ Opera 12 @ flash
- youtube @ cyberfox @ html5

When I attach a (way more expensive but much worse) usb keyboard simultanously, the key presses on that usb keyboard don't trigger that "stuttering".
Also key-press-simulations performed by AutoHotKey do not trigger stuttering.

I therefore think about a very low level problem in the ps/2 keyboard drivers or some interrupts or so...
And I've thought maybe someone in this forum is familiar with issues like this - I think they can occur on other operating systems, too.
(But XP and 7 were fine for me.)
I think it is related to the missing chipset drivers not aviable for my board for win10.
At the moment I'm using a ps/2-usb adapter to connect it to a usb port, which makes the problem disappear.

But these missing drivers may be the reason for many other problems, like especially: weak performance (esp. on old games), input-delay, ...