No fault codes rules out the coilpacks. The ignition amp uses a heat sink, silver plate on back of it.
Its going to be the amp getting too hot. You could try re-use the amp you have, buy some heatsink paste to apply to back of it and make sure plate is still on back on it onto airbox.
You may have to replace it though. I know i have at least 1 in garage or buy a new one.