Filtering supply lines

[Brian]

>> problem with that probably isn't the positive side of the supply.  I'd check 
>> your ground.  Being a veteran with car audio I can tell you the horror 
stories 
>> about automotive grounds.  They don't always stay at 0v, EPECIALLY when when 

>I tried to eliminate that problem by running all my circuits grounds
>to the same point, and connecting *that* point to ground.
>That *should* eliminate the problem, right?
No, this won't eliminate the problem if it is with your ground.  Just because 
you connected all the grounds to a single point all you are guaranteeing is 
that all those points will be at the same potential, which may or may not be 
0v.  Because the entire chassis of a vehicle is used as a ground you have 
variable resistances between points and often get various potentials on the 
"ground path"  The only way to guarantee you are at a true 0v ground is to run 
a wire back to the battery terminal.