One problem it could be is the piston/cylinder fit has expired, the engine has enough compression to start cold but as it warms up, the cylinder expands much more than the piston and the compression drops off too much, now it has almost nothing to keep the process of charging the next mixture and compressing the mixture to keep it going. Eratic idle settings are a dead give away to a worn out piston/cylinder.
OR
You could have to enrichen it a bit to see if your too lean at idle, make sure the idle stop screw is adjusted properly.
OR
You could have an air leak that is effecting you idle after warm up, sometimes an O-ring is tight when the engine is cold, but when the case warms up the fit expands and the O-ring no longer seals tight, common on flattened 0-rings.
My Mach 15 had the same issue, it turned out to be the O-rings were shot and ity was leaking at the pull start shaft, it relied on the seals on the internal one-way bearing to keep it sealed. I modified the rear cover plate to accept an aditional O-ring to eliminate the chance of it happening again.