Secondary Flutter
I had this problem! Pin them open and see if the problem goes away...

(possible fixes)
Disconnect the IMRC plug (clean if necessary) and pack it full of dielectric grease and re-install it.

Hand operate the secondaries to full travel several times. They could be having travel problems due to crap build-up inside them (See FAQ on cleaning)

IMRC Replacement. (okay that's obvious eh?)

Bad plugs and/or wires
How new is new?
~Check them - that was simple... wink

2000 SVT #674 - Check it out!

Whoever coined the phrase; "If it ain't broke; don't fix it" ~ Just doesn't get it...