Do you really have an "amplifier" connected. If so, bypass it, and bypass the splitter, then check if the symptoms change.
How many coaxial-cables have you swapped:
- wall-outlet to amplifier,
- amplifier to splitter,
- splitter to TV,
- splitter to cable-modem,
?
A splitter outputs less signal-strength (on each output) than what it receives on its input port. Perhaps, your incoming signal-strength is "marginal", and dropping-it, through the splitter, is causing your symptoms.
I doubt that changing the splitter will be the remedy to your symptoms..
Instead, contact Shaw (online-chat or telephone) and get them to remotely logon to your cable-modem, when the splitter is active. They can check the signal-strength entering the cable-modem. Then, ask them to wait a minute, while you bypass the splitter, and get them to re-check the signal strength (and other values).