"The car had been running great and not to rich since I’ve installed the latest plugs 15.5-16.5 AFR on idle" - that's not normal. Normal ratio is 14.7:1. Running this ration on a bank will result in some cylinders actually being much leaner too as the ECU is trying to get to 14.7:1. It seems to me like you might be overheating the plugs and that is what might be causing the gap to close.