[GreenKeys] 33 paper tape punch problem

[Charles]

I haven't punched any tape with my 33 for a while. Today I discovered an odd 
malfunction.
While punching, every now and then the tape will advance without punching 
anything (including the sprocket hole!)
Eventually it will advance more than one location, at which point the tape 
buckles inside the mechanism (upstream of the punch pins) and jams.

Here's a pic while it's still punching continuous rubouts but hasn't jammed 
up yet:
http://s1181.photobucket.com/user/DrCharlesMorris/media/PDP-8/P09-16-15_18.31_zpstvbq4fov.jpg.html

I originally thought the tape was binding, so I oiled the punch pins and the 
roller at the rear of the tape feed trough. Didn't help. Then I realized 
even if the tape were binding, the holes would be punched too close 
together, not too far apart, and there would still be a sprocket hole 
punched too.

The typing unit is showing correctly what should be coming out on the tape 
when I send printable characters, and it's not missing a beat (distributor 
steadily retriggering). Besides, the punch is mechanically slaved to the 
codebars so its levers have to be moving the same as what's typing.

Before I adjust anything (and yes, I have read the manuals), has anyone seen 
this before?
I can't figure out what can cause extra feeds without punching.
thanks
Charles