[Healeys] missing at high rpm

[Maurice Maxwell]

After replacing a head gasket and all of the other gaskets in the kit and having 
the head rebuilt.  I tuned the car and it drove around town just fine.  Steady 
idle, smooth power.  However, on the highway, there was a serious misfiring at 
about 4000 rpm when the throttle was fully depressed.  When the gas pedal was 
slightly released the smooth performance returned and the car continued to 
accelerate on to 5000 rpm.  I find this to be strange and I need help figuring 
this out.  The car did this in 3rd, 3rd o.d., 4th, and 4th o.d.  It did 
accelerate on to 5000 rpm in each gear when the pedal was not quite on the 
floor.  

Thanks for your help in advance,

Max
1961 BT7 MkI