My first consideration would be secondary ignition.... under normal
conditions, we wouldn't expect to see required voltage exceed 15KV or so. If
we get into the pedal, required voltage (this is the voltage required to
ionize the spark plug gap) increases. We may find a condition where the
insulation on the wires can't contain higher than "normal" voltages or there
may be a carbon tracking or moisture problem that allows higher voltages to
"escape", following a route we didn't intend.  Another concern could involve
a faulty ignition coil or module that affects coil saturation (this refers
to the strength of the magnetic field when the coil primary is turned "on").
If this magnetic field isn't strong enough, the coil may not have enough
capacity to both ionize the spark plug gap AND supply current flow to
initiate combustion....

HTH