And another thought!
Could be a plugged emulsion tube.
On each side of the vent tube you'll see some holes. These are the emulsion tubes. They can sometimes get plugged and your secondaries will run lean and sputter. You can check them by running a welding tip or pipe cleaner thru them. A steel guitar string works too.
Here's a link with some other possible cures.
http://www.digitalcorvettes.com/foru...ad.php?t=88376