Robert_Charlton - 5:46 pm on May 30, 2012 (gmt 0)
sudden alternating of category page and product pages
IMO, this suggests that the pages are quite close in ranking "scores"... perhaps for different reasons (ie, different sets of ranking factors)... and that Google is testing one set of factors against another. Possibly the results are returned by different data centers, and the suddenness of the switch may be due to your seeing one center vs another.
For a related discussion, see this thread...
Why Do Some Homepages Get Buried?
CainIV makes this observation about pages from a domain he sees switching in results...
...almost as if some type of internal usability test is being done to select the better of the two pages.
...and he and I exchange some comments about what we've been observing in this regard for the past two years or so.
Sounds to me like you're seeing the same thing, only with product pages and subpages. The entire thread is filled with observations that could easily apply to this situation. Linking, of course... both internal and external... enters into it.
PS: Your observation of the pages switching even when you have 100 results selected indicates that they must be very close, as in #20 and #21 (as you've observed), and their rankings are almost a toss-up. If they were further apart, say #20 and #35, it's not likely that they'd switch, as the #35 would be unlikely to pass the #20, which is what it would have to do, to cause them to switch.