i assume this is the machine?
we might need more info...
how long are you letting the machine be "on" before testing the temp?
is your temp gauge accurate? (test in ice slurry and boiling water)
the foam cup is ok but your going to lose temp FAST!! so preheat the cup and temp gauge etc.
Have you been flushing pre shot ? does that make it get hotter or cooler at the Ghead?
apart from ideas, turning up the pstat should result in higher boiler temp and pressure....
what sort of coffee is it, how fresh, and what do you grind with?
(sneaky idea the breville has a pressure basket.....)