What could be causing differential signal drift?
You don't seem to have a ground reference. The scope measures relative to ground.
What is the frequency of the signal?
It could be beating with noise pickup (such as 50/60Hz) because you don't have  aground reference.
You could split the 215 ohm resistor (why 215 ohm?) into two 107.5 Ohm resistors with the junction connected to ground. That would refer the two signals to ground but still terminate the differential signal.