i just recently did a twin plate job on my s13.
according to this turbomagazine and the "cto" of ACT, clutch slippage is usually caused by:
oil contamination,worn friction material (not likely if you just changed your clutch),or lack of clamping power.
have you checked your clutch fluid? how is it shifting between gears?
this is the link:
http://www.turbomagazine.com/t....html
maybe it will help?