I'm not sure what you are trying to do. Have you mixed twk commands and sampler commands in the same track or what?
I mean. The parameter interpolation function just interpolates between the values, and, your example of volume slide would be putting first 0Cxx and at the end 0Cyy (xx and yy being the initial and final values) and then apply the interpolation function.
If this isn't what you meant, please, be more specific or give an example of what's happening.
<[JAZ]> Pa pi pa pa pa pi pa.... ;ĚD