Over rev...HELP
 

52 plate impreza wrx bugeye...standard.
Hi all, I've got a problem with my Impreza wrx,
When I change gear the revs go up by 300 to 400 revs before dropping back down,
I have had a new clutch fitted as it used to have a high biting point and needed changing and I have tried changing gear differently but it keeps over revving in between gear changes.
Could it be a sticking ( idle control valve ) or the ( neutral position sensor ) as I have heard these could cause the over rev, but think it best to ask on here to hopefully get to the bottom of this annoying problem.
Any advice much appreciated.