I have a 2012 Hyundai Elantra that is intermittently hard to start, usually when the engine is warm. When I finally do get it to start the engine sometimes runs really rough, sometimes runs normal. Also, the tachometer sometimes fails to register; though the rest of the instrument cluster lights up. So far I have been able to get the car to operate normally by then turning the car all the way off, removing the key and then restarting it. The fact that the instrument cluster is involved has me puzzled. Also, there are no Check engine codes.
That may indicate there’s a problem with the crankshaft position sensor.
The tach gets its’ signal from this sensor.
Tester
I think you are right. It occurred to me as soon as I sent the question. I’ve been stumped by this for weeks! 
I will replace the part and let you know if the problem returns.
Another idea, try warm-starting by first holding the gas pedal all the way to the floor. If it starts ok then, it means the starting air/fuel mixture is too rich. If that turns out to be the case, post back here for more ideas on why the mixture is too rich. If the engine runs ok at the same time there is no rpm display, seems pretty unlikely the rpm display problem is caused by a faulty crank position sensor.