Just returned from central day

I think I have a problem with the ignition amp on my car, after the engine has been running for a good while, lets say 1 hour down the motorway for example,


The car develops a missfire/back fire always ar 4000rpm, I have upgraded to a gpa coil, new champion qc59c plugs, the cap and arm are new. I have renewed the heat conducting paste for the module.

When the misfire kicks in the rev counter normally drops to zero for a second.

I've had a read on google and a site mentioned cooling the ignition amp with water, this helped whil the engine was revved up in my garage.

Anybody come across anything like this before??

Its on a cosworth managed cvh turbo