I recently installed a CRT distributor/coil in my 55 Thunderbird 292 (stock engine), auto trans.    When I was checking the advance at different RPM's, I notice the timing strobe is not steady at RPM's higher than 2000.   Once every 5 seconds the pulse/light beam stops for what looks like one firing cycle.  Then it's steady for another 5 seconds and it misses again.  Same issue with and without vac advance connected OR plugged.  At idle the everything is TOTALLY consistent. 

Once the car cools off, I am going to put my 57 distributor and coil back in to check it. 

My timing light is an 30 year old Craftsman dial back light that I never had a problem with.  I checked it against a friends SUN timing light and it seemed to match.  Wires are
Taylor about a year old, spark plugs are new.

Any ideas?