this has come up a number of times on this forum, and the most common cause is a failing main battery, although the other possibilities are just that. Might be worth having your battery checked, or check it yourself if you own a discharge tester.
A multimeter set to voltage is not sufficient to test a battery, it only indicates the voltage present, not its ability to deliver a current. Therefore a discharge tester must be used.
Co-incidentally, has this only started since the cold weather came ?