Derailleur twitching in low gears I had a mild spill today on my new bike, which put the (Tiagra) derailleur cage a little bit out of line. With some bending and eyeballing, I got the two jockey wheels lined up 'almost' perfectly parallel with the cassette, and shifting is okay.

My problem is, when I shift down to about the 3rd largest gear and larger, the cage 'ticks' when pedaled about 240 degrees (not in the same spot, just after about that much pedalling). I watched the cage and what's happening is, the chain starts to drift to the right (outside) of the lower jockey, and then, for some reason, one end of a link rises up off the jockey, and the cage pulls back a little bit like it's trying to get in position for a shift. It doesn't go far, though, and by the time the bunched link gets off the lower jockey, it loosens up and the tension drops out of the cage, causing it to relax into position again.

None of this happens on smaller gears. What's going on here? Can I fix it without shelling out cash for a whole new derailleur (was basically new)?
From what you say the chain tension is now too high. 

There should be a screw that adjusts the tension of the tensioning spring. Sorry I can't tell you where it is: none of the images I can find show it. Check out these sources.

Before adjusting anything take note of the position of each screw, then find the one that you think adjusts the tension and turn it one whole turn and test the result. If it messes up something else, it's the wrong screw so turn it back again. If it changes the twitching and clicking it's the right screw, so adjust it until the it's as good as you can get it. Keep track of how many turns you make, in case you need to start again. If that happens try adjusting in smaller steps. 

Edit: see also the links in this answer.