I really like my new Rival group I put on my cross bike, but I've noticed a problem that I'd like to fix. When really jumping on the pedals and mashing hard to get moving quickly, from a stop or really slow pace, the chain skips to a lower gear in the rear and then bounces back to the gear it is supposed to be in.

Unfortunately, I didn't have time last night to look into it more. This weekend, I want to recheck everything and make sure the rear derailleur is indexed properly. But, from a cursory check it appears everything is set up properly.

Any other ideas what I should be checking besides indexing? And I did make sure my fingers weren't touching the shifter when I would demonstrate the problem.

The set up:
Rival shifters, front and rear derailleurs, 1070 11-26 cassette, FSA Gossimar cross crankset (46-36).

This sounds like the typical symptoms of frame flex induced ghost shifting. When you really crank on the pedals hard (as when stomping on the pedals at a low cadence), the frame can flex. The derailleur cables, which are run through stops and guides at a few points on the frame, to get pulled or slackened as the frame is twisted or bent, causing the derailleur to move to one side or the other momentarily, causing the chain to shift. Dirty or sticky cables or housings can increase this tendency.

The first thing to check is that the cables are run correctly, and that they are not catching on anything else: Do the cables rub against some other frame fitting or component? Do they cross or pull against each other?

The next thing to check is the indexing adjustment. Is the indexing biased a little bit toward the bigger sprockets (lower gears)? There is always a little bit of tolerance in the index adjustment, so that it can still shift properly if the adjusting barrel is a turn or so from either side of "dead center". But if the adjustment is bias a little bit the larger sprockets, than it may shift okay during normal riding, but a little bit of extra cable tug due to frame flex may cause it to "ghost" shift to the bigger sprockets. I'd try turning the adjuster toward the smaller sprockets (barrel scews inward, or clockwise) half a revolution to see if that doesn't reduce the ghost shifting.

Check the B tension screw on the derailleur. SRAM recommends 6mm from the pulley to the cassette teeth. If is spaced out further then the derailleur cannot shift well (especially when doing double shifts to an easier gear).

I had the similar problem and adjusting the B tension screw helped. See page 4 of the manual.

http://www.sram.com/_media/pdf/sram/dealers/TM_RoadMTB_MY07_E.pdf

Check the B tension screw on the derailleur. SRAM recommends 6mm from the pulley to the cassette teeth. If is spaced out further then the derailleur cannot shift well (especially when doing double shifts to an easier gear).

I doubt the problem is the B-tension screw. If the derailleur is too far from the sprockets, the symptom is that it doesn't want to shift at all, not that it shifts on its own. And if the derailleur is too close the sprockets, there will be excessive noise as the pulley trys to grind against the sprocket. There is no doubt that the B-tension screw needs to be adjusted properly for good shifting performance, but a poor B-tension adjustment want cause inadvertant shifting.

Just to close things out. I biased the indexing to the smaller cogs and everything is running smoothly.

I also have to admit, I really like the feeling of Rival over my old Ultegra on my road bike.