Magpul midcaps making my gun inconsistant?

[Tomm]

Right i got a bit of a problem brought some magpul midcaps brand new took them out last sunday the 12th worked fine then all of a sudden they fired couple of bbs normal then few dropping short kept alternating like that with every mag i thought to myself my guns broke got home got my chrono out loaded a mag 330fps for 5-10 shots next 5-10 shots 200fps scratching my head tried a high cap 330-340fps everyshot ?
How can the mags make the fps inconsistant i wouldnt be bothered if it didnt effoect the range much but it basically halfs the range? Any one had similar issues any cures
Im using a kwa sr10 m4