No, I have the 2.85 gearbox.  I have the MT electric prop, so I fly WOT and use the prop to govern the engine speed. 

I have changed the plugs with no effect.  The mixture is fine when this is happening.  Fuel pressure also.  I had an incident once on take off like you describe where the plane was backfiring like crazy just after I lifted off.  I had the mixture too rich.  I dialed it back and everything smoothed out.  This is different.  It doesn’t really backfire.  It is a thump and it jerks the engine and plane.  You can hear and feel it.  It seems to be a single thump and it has varying degrees of violence.  Some are just a little miss and some are so bad it shakes everything.  These are less frequent, but they are bad enough that I don’t think they are good for the PSRU, prop, or engine mount.

That is why I have been bugging Tracy about the change for the EC-2.  I have my fingers crossed that this will fix it.  :>)

Bill B