My car has been exhibiting some odd behavior, that I can only assume is the result of a bad ground or wire.

Once in awhile while driving, it'll act as if the battery is no longer part of the equation.

The brake pedal, turn signals, high beams, all will make the radio and blower motor cut out momentarily when switched. Lights flicker as if just off the alternator.

When the car is parked and turned off, the key going to "ACC" results in a completely dead car. Battery is new, reads 12.5 resting.

Now, say I add a jumper to the battery terminals, car starts just fine.

Car has actually been starting fine for the past few days, and running normally. However, due to it being winter, I am holding off on driving it until I have a better idea of what it could be, before I break one or more expensive components. Thanks for any help.