2004 Honda civic ex check engine light?

I m having a code that reads p0 2646 the problems that the car has is when you get to 5000 rpms the vtech doesn t seem to kick in the car kinda shakes so I just et my foot off the gas and slowly get to speed. So just want to know what part or parts to check and possibly what part I need to change. But note that this problem wasn t happening before a head gasket change?

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