Yes, there is a ground bus by the drivers seat under the carpet by the door. Check the techinfo as I believe there is a good article on how to clean it.
The idle fluctuation when cold is normal. It should go to a normal idle as soon as the motor warms up to operating temperatures, which shouldn't take long.
It does sound like a coil is going south, especially if it changes when you tap it. You could check the connections if you want, pull the coil and ICM assembly off and use a wire brush to clean all the contacts.
RIPBC... Off to bigger and better things