Linksys Wireless Router :: E2000 PC Gets Choppy In 40 Mhz Mode

I have a e2000 router and a wmp600n wireless card in my desktop pc.  Everything works fine until I enable 40 mhz for faster speeds in the e2000 router.  When I do this the router switches from 20 mhz to 40 mhz and I get the faster speed.  But the effect of this is that my pc gets all "choppy".  What I mean is that everything freezes up every few sec.  For example when im minimizing windows, the windows freeze half way for about a sec then unfreezes.  My mouse does the same thing, even my sound is choppy.  I have to switch my router back to 20 mhz and then everything is fine again.
 
Using windows 7 64 bit

Using newest dirvers and firmware from Linksys.