Check your connections, make sure everything is making contact as it should. Especially if you're breadboarding.
Add a small decoupling capacitor across the 100k resistor, size depends on noise vs responsiveness, 1nF should be fine for a starter.
DMM probes are around 1 to 3nF, depends on how tangled yours are.
Schematic would be nice.
Could also be noise, i tend to get a lot of noise in my circuits when i have SMPS power supplies running.
Also it could be a grounding issue, especially if the curcuit is being powered from a switch mode PSU.