newbie:memory problem or...?

during the last couple of weeks my system occasionally behaves funny,
mainly the mouse starts jumping all over the place and all applications
take their time before loading and then they function with a " hisscup"
, sorry lacking technical knowledge I do not know how to describe that
behaviour...this is the output of the command "free"

[santo@skynet ~]$ free
total used free shared buffers
cached
Mem: 483316 478040 5276 0 964
34444
-/+ buffers/cache: 442632 40684
Swap: 1044184 989444 54740
[santo@skynet ~]$

I check with MCC and at some point I can see the swap being reduced to
almost nothing and Application data squeezing "disk cahe memory" to a
thin green line and the "free phisical memory" basically disappearing.
This behaviour is random, at times the system works fine.

I am using Mandriva Power Pack and Slackware, but this problem never
occurr when I am in Slackware...
I do not know what to think...any help available?
santo