Whew! A lot going on! 12 hours a day for a 5K pool? That's about 4 times longer than you need--I'd run that size filter on MY pool and it's 20k. Are you running at low speed? is that what you meant?

Now the problem is almost certainly in your multiport valve. I'm not the MPV expert but you symptoms sound like
a) A perforated or broken wall between channels in the MPV
b) A damaged and/or out-of-goove spider gasket.
c) A damaged valve mechanism.

That's what I'd check first.