Ummm...

In my experience, an erratic or inconsistent spark is something in the distributor. The coil either works right, causes a weak spark, or doesn't work at all. The one exception to that might be if it works for a while, heats up, becomes intermittent, and then stops working. But the coil is an amazingly simple and robust part compared to all the fiddly, moving, flopping and wearing parts in a distributor. So while checking (swapping for known good) coils isn't a bad idea, try the dizzy, too!

Personally, I'm running a blue coil with the Allison (now Crane) ignition unit, with no problems. But I've also heard that this isn't the 'best' setup....

Toby