Originally Posted by
HillRider
10-speed cassettes are very closed spaced and pretty intolerant of "near miss" shifting so a wider spaced cassette should help. 8-speed would be best as it has the same spacing as 7-speed but offers one more cog. ...
As others have mentioned, auto-shifting like yours is a a trim problem. On index systems the trim is built into the shifters, without index it's up to the user. You're using a very narrow cassette which has shift gates which make it more prone to shifting than plain sprockets would be.
Start by checking that all housings run cleanly and squarely to the cable stops. The least amount of housing flex at a stop, levers the ferule against it's rim pulling on the inner wire and changing trim. It's possible that flex, bouncing or rocking as you claim is moving the housings enough to throw the trim off.
Originally Posted by
HillRider
...You mention it usually occurs while under a lot of pedaling pressure particularly while standing which does indicate the rear triangle is flexing under the high chain tension. The 8-speed cassette will allow for more alignment slop but a stiffer frame is the only sure cure.
There
may be another remedy. Using a floating pulley for the jockey wheel as in Shimano index derailleurs will allow the pulley to stay in trim when there are slight movements of the RD (This is why Shimano uses it for index in the first place). It will take a bit of getting used to but once you do, it might be enough to solve the auto shift problem. No guaranty, but it's worth a shot since a pulley is much less expensive than a new frame.
Note- Replace both pulleys with a matched set floating upper/non-floating lower, since there might be a difference in overall width between the new one and our old ones. You might also need longer cage screws, so check that there's enough thread engagement and buy longer screws if needed.