"Flashing Flo" or "Solid Flo", are two differrnt signals to the circuit board. You should be able to see the flow of water actually close and the magnetic finger switch, when it dosen't close completly = "Flashing Flo", when it does NOT open it signals that to the circuit board = "Solid Flo" and nothing works.
As for 2 temp sensors, they should agree within a few degrees of each other, IF NOT = watchdog, or (----) on the read out panel and nothing works. A test plug would be real handy here.
When you get the "Flo" message, verify the circ pump is actually moving water through the flow switch. I have witnessed flow switches that appear to OPEN but the circuit board says something different and it remains a secret until you check continuity between the two wire leads. This is what the switch is telling the circuit board and may not be seen by the eye.
When you have the problem, power the system down, remove the sensor loom from the circuit board, (it may not be open to the circuit board) and then put it back on and power up the spa. This will duplicate an opening of the switch and it will close when the circ pump starts up reverting to normal operation until shut down again.
Could be a bad flow switch or a failing circ pump, put your hand on the body of the circ pump and see if it is "very hot" to the touch.