What atomiser are you using on the mod - and, if you have another mod to try it out on - does the atomiser you're using work properly on that mod?
It is frequently a connection problem that makes the ohm reading jump around,
Try cleaning the 510 on the mod and the bottom of the tank and make sure both are clean and dry. If you are using a rebuildable RDA/RTA make sure all coil legs are firmly held in position - a loose screw will give you constantly different ohm readings. If you're using a stock coil tank, strip and clean it - paying particular attention to the positive contact and remove any baked on juice deposits on the bottom of the tank, before re-assembling. Hopefully this will help.