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Look at the sprocket teeth with a magnifier or pocket microscope (Radio Shack sells a nice illuminated 30X pocket microscope). Replace the worn/damaged sprocket. In a pinch, sometimes "reversing" the sprocket on the shaft will work as a temporary fix, since the other (undamaged) side of the teeth will now be driving the film. Fix it soon, as worn or damaged sprocket teeth do damage the film perfs.

The same "clinging" behavior can sometimes be seen on other worn sprockets. The snapping/clattering sound was sometimes referred to as the film "singing on the sprockets".

Check your gate and sprocket shoe tension/alignment/adjustment, and be sure the film tension is as low as possible while still maintaining a steady image. Excessive tension wears out both film and sprockets prematurely.

I would even venture that reversing the sprocket to run on the other side of the teeth is more than a temporary solution, as long as the sprocket is in otherwise good condition (noo nicks, burrs, etc.). You should get as long a run on the other side as you did on the first. Be sure it hasn't already been reversed, of course...

Have you changed an intermittent sprocket before? The challenge is in its alignment. On a Flat (Straight or "Studio) Film Trap, use a proper alignment gage such as a LaVezzi AG-1.

It's a little tricker with the curved Trap, though the procedure is outlined in the Operator's manual as I remember. Use a small straight-edge off of the outer edge of the lower holdback sprocket to locate the intermittent sprocket to the trap. The outer edges of both sprockets should be in line.

Steel film is an alternative with either of the above.

Good luck!

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Let's see: 21 years of 5 two-hour shows per day is:

21 years x 365 (days per year) x 5 shows x 2 hours x 60 (minutes per hour) x 90 (feet per minute) = 413,910,000 feet of film, or enough film to circle the earth's equator three times!

Yep --- time to get a new sprocket

I have seen projectors where the pad shoe gets bent or somebody didn't put the thing together right and the shoe grinds up against the sprocket, wearing the teeth down.

I wouldn't say it's a "commom" nistake but it's also not UNcommon. Sometimes people just make mistakes...

PS: Your projector is ONLY 20 years old? Heck! It's still just a young 'un! I had a projector that was at least 40 and it made a better pic than a brand new one! (That was back when they knew how to build 'em! )