I have a 383 stroker with the mercruiser thunderbolt IV ignition. I started a miss in the upper rpm range. sometimes it might start missing as low as 4300rpm sometimes higher. Thats the only time it misses. It pulls really strong till u start getting in upper rpms. Ive checked timing,changed fuel filter and water sep. plugs cap roter all in great shape. Fuel pressure is good thru out rpm range as well. was wondering if this is a symptom of a module going bad. At over $300.00 i just dont want to throw one on with knowing if its bad.

I would guess you have tired or broken valve springs. I believe the modules either work or don't work, no intermittant stuff.

How old are the ignition wires?

I highly doubt the module is bad. Maybe a weak connection at one of the wires that exposes itself under a heavy load.

Try changing the plugs and wires. Then try changing the coil.

I had a module go bad last summer. It would either start or not start.

this is a new motor,probably have around 35 hrs on it. The only original parts on the motor are the block and the ignition. One weekend it ran great the next its missing on the top end. Ive got a spare coil i can try this weekend. Also im going to check the neutral cut switch and make sure its adjusted right

I had a merc 4.3l that ran good up to about 3500 rpm. Then after that it would stumble and miss bad. It was a bad module(found out the hard way). 12v test light on secondary side of the coil showed erratic light above 3500rpm. as a side note, on this boat if you hit the trim button it cleared up and ran good. you can substitute any thunderbolt 4 module (6 or 8 cyl) as the only difference is the advance curve to test only. hope this helps