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Just a guess but check your water level. If the level is a bit too low, the pump will slowdown because it is not getting enough water returning through the filters. :-/
The diverter valve, if it's like mine (2003 Optima) divides water flow between the whirlpool jets, and the jets in 3 of the seats. On our spa, that'd be powered by the 2 speed pump, which pulls thru the main filter. Your may be different. What year is the spa?