Help Required For Tachometer Circuit...

Hi,
I am trying to build a tachometer using IR Sensors..
I get good response at lower frequencies but as the frequency increases say to 170Hz it looses the wave..
i am using a hysteresis resistor of 1M at positive feedback and input is at inverting terminal of op-amp should i use hysteresis resistor i think the problem is with this as the frequency increases..
and if i remove the resistor response is so fast and ir sensor is imune to even little jerks..
Plz Someone give expert opinion.
Thanks In Advance

A picture is worth 1000 words.
Can you give us a schematic?

Which IR sensor are you using? Can you post a link to its datasheet?