Consistent Encoder RPM Issues

[meltbox360]

Hi all. I am trying to figure out how to get encoders to read consistent values for speed. We have 250 count encoder (e4p) on there and using getRate() they read correctly at low RPMs but once it starts hitting 6000 rpm then we start seeing variations of around 15 revolutions per second. These are mounted on a mini cim. I don't really know what to do at this point. I have tried varying them from k4X to k1X with no real difference. My other thought was doing some kind of

reset
wait
get elapsed ticks
get elapsed time
calculate rpms

as I am under the impression that the variation could be due to the miniscule time frame between ticks that WPIlib uses to calculate the rate? I couldn't figure out how to spawn a new task to do this is and I do not want to block my code. Essentially I have run out of experience and I couldn't find much in the way of code examples or a clear solution to my problem.

So here is my goal:
Get a clean rpm reading useable in PID or other algorithm to ramp up a frisbee shooter to the desired speed.

My problem:
Highly variable readings at higher wheel speeds.

Thanks,
Team 4302