bc,
My best guess is that you're running that on 110v in which case you're dealing with a smaller body of water that shuts the heater off when you turn the jets on high. This being the case your cooling the water with the jets and with your body temp taking up a large amount of the volume.
I suggest running 220...If you're on 220 and still dropping below 103 when you turn the jets on then you might have the pin for 30/50 amp set wrong...