I'm hoping someone can help me with this rear derailleur indexing problem that's driving me crazy.

Some background: I'm using SRAM Rival rear derailleur, Rival shifters, SRAM chain, SRAM 11-28 cassette, and SRAM Red 38/53 chainrings and crank.

The problem: The rear derailleur keeps going out of index in an odd way. It will be indexed perfectly for while, and then after maybe 30-40 miles it will suddenly no longer stay on the largest (28t) cog (not cross-chaining - happens while in small chainring). It will shift into that cog, but it will immediately drop back into the next largest cog. This then throws the entire rear derailleur out of index because the shifter is in its lowest click but the chain is on the second-lowest gear. Rather than self-correcting on the next shift, the whole system then stays one cog off as I shift up and down across the cassette (though it's not perfectly one cog off - it's probably about 3/4 of a cog off, causing the chain to slip and rattle all over the place).

The indexing will have been fine until this happens - it's shifting onto the cog and having it slip off that pushes the whole thing out of index. It's then so far out of index that it cannot be fixed using the barrel adjuster. I have to re-adjust the tension in the cable and re-index from scratch.

The first time this happened, I figured that the cable must be slipping in the rear derailleur pinch bolt when under the greatest tension (when shifting into the largest cog), leaving the indexing out of whack. But I've gone through this whole cycle several times now, and I've measured the length of excess cable coming out of the pinch bolt and it's not changing. The pinch bolt is firm.

Over the past week, I've had the derailleur indexed by two different professional mechanics, as well as doing it myself. It slips out of index in the same way each time, after maybe 30-40 miles. I just re-indexed it again, and once again it's fine, but I expect the whole cycle to repeat itself.

I've had this cassette, chainring, and chain for about six months with no problem until this started a week ago. Before this, my prior components were the same size and make, also with no problem. I've had the derailleur for two years. The limit screws and b-screw are in the same adjustment they've been in for two years without problem.

Any thoughts are much appreciated by both me and my wife, who has had to deal with my obsessing for the past week.