If your graphics card shares memory with main system memory that's a
potential cause. Many systems today use this type of a setup instead of
using graphics cards with their own memory so that's one thing to check. If
that turns out to be the case, no amount of tweaking the memory settings is
likely to fix this as the more memory you allocate to the card, the less
that would be allocated to main system memory.
Another possibility are applications running in the background. Antivirus
software can be enormous memory hogs, worse, if the application is sampling
or otherwise checking data stream of the clip being played, that could be
the issue. You might try turning off such software during the playing of
clips and see if that resolves. NOTE: if your graphics card doesn't have
its own memory, while things may improve, this is not likely to resolve the
problem