Can you try adding extra "wires" sa negative ng battery and attach it to the car's body...make sure metal contact (isand paper mo kung kinakaillangan)....it looks like a body ground problem.

Typical symptoms yung sinabi mo ng kulang sa body ground or a bit of a loose contacts sa ground...tumataas kasi ang resistance habang loose ang contacts. As you could see may improvement na when you tightened the bolts of the body ground.

or try mo mag lagay ng wire (lakihan mo ng kaunti) na direct from the battery terminal ground to the body ng radio mo....then test it again.

just my 2 cents.