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Boiler Blues :(

smithfan Member Posts: 91
Triangletube Solo 110
4 circulator pumps. 1 inside boiler, 3 managing zones.
2 baseboard zones, 1 in-floor slab zone, preceded by mixing valve.

I'm at the end of my rope trying to figure out this boiler problem. I've attached pictures. Basically once or twice a year the Baseboard(zone2) circulator will stop pulling/pumping water into the zone. It's a grunfos 3 speed and is still running when this happens. This winter it's already happened 3 times, so the problem seems to be getting worse. I've fixed it 2 different ways. One way has only worked once, the other way has worked every time.
Yesterday with both baseboard zones calling, I turned the pump speed of the problem zone from low to medium and then it did pull the water through and heated the zone. Almost like a magnet was holding it back!
However, today this didn't work. I came home and no heat again in that zone. I turned off all other zones so it was the only one calling for heat, and it still wouldn't pull through, instead my boiler overshoots it's setpoint believing there is flow through zone 2 ramps up past 200 degrees because of the super hot primary return water. This can't be good!
The only full proof method I've done to fix this the problem is isolate zone 2, turn on the fast fill and purge the thing..Then it works fine again, almost like it was air bound or something.
The only thing I can think is my piping arrangement in my secondary loops have too many 90 degree bends, or there is some flow issue. I'm open to any ideas. I ramped up the boiler pressure to 20 psi from 15psi to possibly try and help move any air out of the system. I just don't know where to go from here. I am an 1st year HVAC student, unfortunately Hydronic Heat isn't until year two!