DFRobot_RGMmatrix library and a serial port

Has anyone successfully added a serial receiver to a sketch that is already using the DFRobot_RGMmatrix library? I can receive characters but they are garbled. Same code works if I dont call the matrix.begin();

So I am thinking I have a interrupt problem or that the DFRobot_RGMmatrix is gobbling up all the CPU time. Thoughts?

For the serial library, I am using AltSoftSerial.

Thanks, Jeff