Slow QAM playback

I recently gave my sister my first HTPC setup. She purchased a USB tuner to add to he Diamond 750 PCIe tuner installed. The QAM playback in WMC is slow and glitchy especially with HD channels. Any suggestions? I suspect the USB tuner may be at fault. Is the a way to set the PCIe tuner as the default or might something else be a possible solution?

Might the issue be too many splitters before the tuners?

live tv only.

Unfortunately it seems the internal tuner is dead. It isn't old, going to be sent back for a refund. The cable signal itself is poor. She gets random pixelation on HD channels. I think we are fighting two battles. Poor signal and poor tuner.