Ignition Check Irregularity

912ULS 1050 hours flown since 2012 RV-12 South Texas Always hangared

I check ignition modules before and after flight.   Started getting rough on one side, sometimes before, sometimes after.  Kind of a rolling RPM rough, smooth, rough.  I had replaced one module which died a couple hundred hours ago, so ordered new one to replace the one with over 1000 hours on it.  Installed, preflight check OK, postflight OK.  Today, preflight was OK, postflight back to the same old same old.  Exactly the same, rolling....would really get your attention if you switched off the good side in flight.

OK, maybe the newer module was the bad one, maybe a wiring issue, bad plug?  Would appreciate your constructive thoughts on this intermittent problem because I'll bet I can't get it to act up as soon as the cowl is off!  Temperature has been pretty mild here in Texas so I don't think that's an issue.  All flying south of 3,500 MSL.  Ideas?