Could be the RPM signal wire going to the engine computer, which on my car is further away, than the EZL and RPM dash gauge is (computer to right of passenger's feet).

So maybe a poor connection on the RPM wire going to the engine computer or a weak signal - strong enough to get to the closer devices, but not strong enough to get to the computer?

I can read my RPM signal on the round X-11 connector on the driver's side of the engine. As I recall per the factory manual, you can check the "strongness" of that signal with an oscilloscope.

I would clean all connections, especially those going to the EZL.