First check the cable splitter, did you get it from Shaw? If not, replace it with one from Shaw,
Is the splitter hooked up the right way, In from the wall, Out to the modem and Out to the cable box.
If that doesn't help, hook the modem back up the way it was, contact a Shaw support agent to check the levels, confirm the levels will still be ok if the Receive level does down 3.5 points (db) and the transmit goes up by about 3.5.
-Posted from my phone.