Problem with the NV Slider on my MPC2000 - Please help

I can't quite get my head around the note variation slider on my MPC2000. What happens is I'll assign the slider to control pitch for a sound, but then when I adjust the pitch while playing the drum pad, the pitch adjustment kind of does it's own thing- for instance, if I put the slider all the way up, the sound will play at a high pitch for a while, but then gradually come down to a low pitch without me even moving the slider. Then if I keep bangin on it, it might pop up again, but I can't see any rhyme or reason to it. Can anyone help me out? Thanks,

Eben

I don't even know if my sh*t works. Tried it once and never messed with it again

So do you not mess with pitch at all now, or simply program your pitches without touching the slider?