The Paste is a vital part of this aasembly as it transfers heat from the amp to the alloy heatsink,you say your miss is while reving the engine in neutral,Is it like this always or just when a bit warm????

What Plugs you using and what they gapped to?????

When did the problem start??Had you changed anything??

Remember a NEW part can also be faulty so never assume cos its been replaced it is ok

Paul