... not trying to mislead you, but there could be a small ... maybe 1 uF decoupling capacitor across the microcontroller between V+ and ground pins. The decoupling capacitor will pass any spikish sort of voltage waveform to ground, and could make a difference. ... Just a 1uF non-polar capacitor would do the job. ... If you can look at the microcontroller voltage with a scope ... can't say for sure if anything would actually show up, since scope probes have built in capacitance that could influence the reading. ... Maybe just try a 1 uF non-polar capacitor between the micro-controller V+ pin and it's ground pin as an experiment.