Yes, rowing out.
You can use 2 different needles to help yourself to determine which row is the problem..
but the real solution is not to knit with size 6, and purl with size 5--
its to learn to control your tension on the purls.
1--just do it
2--learn to knit differently (knit english? learn to knit continental! Knit continental? learn the norwegian purl! Know how to norwegian purl? Learn combo knitting too..
then go back to your normal style.. just learning all these other methods will help..
(because what you need more than anything else is practice!)
Really you'll find that just KNOWING you tend to purl looser is the first step to better tension with your purls.. (and less rowing out!)
Combo method (my prefered way to knit) is PHYSICALLY easier, but MENTALLY harder.
it requires you know a stitch, (a knit, a purl) and that you can immediately 'recognize' how it is mounted (front leg forward or rear leg forward)
it requires some attention to decreasing..--since it requires you to reverse decrease directions (direction say K2tog? with combo, you do something like a ssk. directions say ssk, you K2tog!)
But its much easier to work a combo purl, and many times easier to control tension/gauge and to not row out..
So.. the answer is easy.. PRACTICE! and continue to learn! (d'oh.. i know, you wanted the quick fix.. but there really isn't any quick fix!)