chain getting stuck between chainrings I have an old Cannondale road bike with Suntour shifters.

When I shift from the front middle ring to the small inner-most ring, the chain falls into the gap and binds tightly between the two rings. The chain seems to be an appropriate width for the back cassette and there doesn't seem to be anything that I can tighten to close the gap between the two rings (the two are screwed together and seem tightly affixed to one another). 

Aside from not using the inner-most ring, I'm at a loss for what to do. Any ideas (aside from a fixie)?
As I see it, from my experience you may have any one or a combination of the following issues (from most to least likely):


You keep on pedalling at force while shifting down. Usually, when you switch from the middle to the smallest ring on the front you are going uphill and you have to keep pedalling just to avoind falling over. Still, you have to significantly reduce the pressure you apply to your pedals while shifting. If this happens often you should try to change gears a bit earlier in the climb when you still have enough momentum to pedal without force for one or two strokes while your chain switches.
Your front derailer is not properly adjusted. Over time the adjustment of your derailer may shift a bit due to al the shaking, or due to dirt lodging between parts. It may need readjustment. If you don't know how to do this I would strongly suggest you ask your LBS for help about this.
Your rings are old and worn. Your rings have special grooves and notches on the sides. They are not there because they look good or save material or weight. They are there to help your chain catch onto the next ring. If the rings are very old these grooves and notches might be worn away too much. Ask your friendly local LBS what they think about your rings or just compare your rings to those on a newer bike.
Your front derailer is old and worn. Over time the different parts of your front derailer are subject to normal wear and tear and everything becomes more loose and you get some play. You can compensate for this a bit by readjusting your derailer, but eventually replacement is the only option.
Your chain is incompatible with your rings.  You say it looks compatible with the back cassette, but in the past someone might have changed the back cassette to a model that requires a very thin chain that is incompatible with your rings. Possibly to increase the number of cogs in the back cassette (and thus the gear range) without changing the rear wheel. Are back cassette and front rings the same brand? Can you find serial numbers? Can you find anything in the website of the manufacturer? Again, you might want to visit your LBS for help on this.
Your chain is old and worn. If you live in a very dry climate and are a master of bike maintenance this might be the problem. But usually a chain is rusty and squeeking years before it is worn too thin.