The Jenkins jobs for the 130K overflow tests used to be extremely consistent. After build #3299 it has become highly variable. The changes in that build are:

 1. WT-3657 Use saved update size for splits, don't grow raw buffer. (#3735) (commit: 6c00bd4) (detail / githubweb)
 2.  WT-3553 Change testutil_die to abort rather than exit (#3731) (commit: 5b3e661) (detail / githubweb)

It seems likely that the changes for WT-3657 at changeset 6c00bd4 are the culprit. I have attached a screen shot of the plot (before it rolls off the chart).