If the RPM dropped 4-500 RPM then there was no ignition in one cylinder, it is most likely that this is due to plug fowling.
The most likely soloution to the problem is to change that plug.
What would be in the back of my mind would be why the plug on a newly overhauled engine fowl?
Could you have a piston ring problem due to incorrect break in procdures ? This would cause high oil consumption and be ther reason for the plug fowling