How many grounding points are there on the C3 ? I have a Hum from my stereo  ( even when it is off ) and an intermittant loss of spark that I am told could be a result of a bad ground.  

I completely rewired my vet  & installed a ground strip on the inside of car attached to the firewall. I ran ground wires from there to anything that need it. That strip is attached to the frame. The main fuse panel has no ground wires that I know of only hot wires there that's why it's fused. Check your main grounding points

No such thing as too many grounds on a Vette or any other fiberglass car.

The "loss of spark" issue will not be ground related, unless the + side of the system is getting grounded somehow. The ign. system will ground thru the dist., and even with an aftermarket system, such as an MSD, those have their own ground wires, so that wouldn't be related to a hum noise thru the sound system due to a bad/missing chassis/body ground.

How the heck could you hear it anyway with that monster runnin?

I'm with 'Slinger...never have too many grounds. I think you might just have an alternator issue, or, if you are running some type of power amp for the sound, I would be lookin at that. If the stereo head unit is off, it shouldn't be able to be the source of a noise. A power amp, on the other hand...

Just play CDs.  I've got no sheilding and I just don't use the radio.  When my tops are off and the rear window out I can't hear anything but pure music anyway.

If it is humming even with the car off, then it is probably a speaker wire.
Scott

If it's a fairly high frequency hum, rather than a buzz, put the biggest capacitor you can find from the alternator output on one terminal, then ground the other cap. terminal to the engine block. I did this on a fiberglass houseboat and the hum went away forever....

Dave

I am going to run extra grounds to my amps, I have three and see if it helps. Also could the intermittant spark be from a faulty key switch ? I replaced the entire column last year which included a new switch. I think I will replace the starter with a new high torque one. Any suggestions in that regard and will a new one ( mini ) just bolt right up ? 

Not sure about the key swich , but an easy way to check is after you start it run a wire from the alt + to the + on the coil with alligator clips & see if the problem goes away. You won't be able to turn the car off until you remove the wire. The mini starter will bolt right up & you'll never go back. You might have to shim it but they come with good instructions.