ST Pumps do not have a temperature sensor.
Yes you should have bought a twin pump. or two singles.
You might not have enough flow when trying too mix the water together, Turn off pump at electrical switch, Get a bucket, run shower till hot, turn shower control full hot, measure flow (place bucket under shower head, time 1 Min) , if its less than 1 litre per minute then you do not have enough flow to activate the flow switch in the pump so the pump will go on/off the pump will get confused.
Although st pumps will activate @ 0.7 liters per min you will never get 100% hot from a thermostatic shower valve.
thanks for the advice, im going to sell this one and buy a double,
As long as you can now get a good mix of temp and its stable then there is no fault with the shower valve.
To mix your in low pressure cold with your high pressure pumped hot, The shower valve will try to slow down the hot flow to acheive the desired temp which will be over powering the cold. as it slows down the hot you will be lowering the flow rate possibly causing the pulsing.
By doing the test with the pump off electrically you are seeing that you do have the flow rate to turn on the pump on normally. so If you have the flow to turn the pump on then you need to pump the cold side. a, Get a twin pump or b, Get another pump the same as you have for the cold side.
*The hot water should not exceed 65 degrees.
Good Luck I will be waiting for that drink.