That pump uses an internal 'bypass' to control flow. Idea it for it to not short cycle On/Off when demand is low.
Pump is coming on/running but bypass is closed at first then as pressure builds up the bypass starts opening (at 40#) and then is fully open before reaching pressure set point (55#)
Like posted earlier......cold/hot have to be mixing for water temp fluctuations to occur out of COLD side of faucet
Is it just out of one faucet or does this happen at other faucets?
If not water heater bypass or showers and only at one faucet then that faucet is mixing cold/hot internally.
If other faucets involved then WH bypass or showers.
