Right, I found that a new battery and a new motor is a must. I am acctually going to finish up making a video on jow to mod your Mp5k PDW/ Mp5k to use a 9.6 stick battery. Most say that you have to dremmel stuff out but thats not the case! There is actually a very easy way to mod it...
Here are more videos-
YouTube - Galaxy Mp5k PDW Review 1
YouTube - Galaxy Mp5k PDW Fireing Test
(Firing test video)