recordings jump back

[pinnerite]

I am using Mythtv version 29.1.
From time to time when a recording is being replayed it will jump back.
I can move it forward again with <number><right arrow> but will just jump back again.
Yesterday I ran optimize-mythdb.pl.
It improved one episode I had been watching but failed on the second.
I is there a method of reindexing if that is the cause of the problem?