The wire you used is probably ok, it does sound like it is heaver then necessary. if you shunt is only 75mv then you should be able to use finer gauge wire.
If you can not get us the manufacture name and model numbers of your equipment, then we would really only be guessing.
My only guess at the moment is that the meter and shunt aren't well matched for your 30 amp controller. In other words your setup is looking for a higher amp draw.
But that is only a guess.
Favorite Quote: "This is L.A., sugar. There is no 'over the top." --- Chris Erskine
Current build: Liahona w/ cheap front suspension and suspension seat post. Yescomusa 36v 800w generic front hub motor. 15ah Headway triangle mounted pack. Tronsung 30 amp,
Previous Build:1992 Trek Antelope 800 - Bone Crusher (no suspension) - Yescomusa 800 watt 36 volt front wheel kit. Don't do it! Get suspension!!!