The usual quickie test is done in the pitch dark. With the engine running, lift the hood. If you have positioned the car so that the under-hood area is in pitch black dark, look at the ignition wires. Leaky wires will have little blue flickers here and there as the high (40,000) volts escapes failing wires. Oviously check #2 extra varefully.

Look also for leaks of exhaust around where the exhaust manifold bolts onto the head. Seems unexpected but I have heard such a leak sound like a click.

While that plug may be firing OK at lower speeds, as compression pressures the voltage must be higher to continue to fire.

The sound. Can you tell if it seems deep from the bottom of the engine or up at the top? At idle, does it click at half the engine speed? That would point to a valve problem.

Do the after-dark tests and post back.

Good Luck,

Bob

:>)