Thanks a lot Calamity, BTW I forgot to mention that when the framerate drops happen sound is distorted as well (as expected)
with that I went and checked a number of things;
- tried an older GM build (0.170) up to newer (0.188) > same result
- reverted from portaudio latency 1, to auto latency 2.0 > same result
- suspecting Win Defender (btw im on Win 8.1) I installed Avast > same result
- made sure no buffering is forced on GM in the nVidia control panel, leaving it controlled by application > same result
That's really strange, I'll investigate further and try several other off-60Hz hardwares and see if I notice something.
I don't know if my memory's wrong but I thought I remembered that some time ago every game falling under my defined sync_tolerance ran butter smooth on that very same laptop.
Arf, mystery.