Tachometer fluctuating above 4000 rpm

Tachometer is old Smiths type designed to sense on ignition coil at points side. Present setup are Vems IGBT output with pullup resistor and inductor, connected to tachometer. Reading ok to approximate 4000 rpm then tachometer starts to flicker as rpm rise.

How are tacho pulses from Vems, are the pulse length constant and how long?

Any one designed a circuitry where pulse from Vems are used to connect direct to instrument (100uA full scale)? - must be possible to design small circuitry there can be placed inside old tachometer there can covert pulse from Vems to current to instrument, suggestions are most welcome.