Why is the Miller welder voltage dropping and increasing during weld?

Welding .375 thick material with .035 wire. Suggested voltage and wire speed is 19.0 and 310 by Miller. Lately during welding voltage will increase or decrease rapidly causing the welding process to snap crackle and pop. I have been welding for a few years and am aware of the settings and how metal heats and you must adjust as you go, but I have never had a problem like this. Does anyone know why this happens? Voltage will drop sometimes down to 16.0 which is a drastic drop in the middle of welding.