This problem may result from several different X server bugs. You may be able to avoid it by switching from overlay graphics mode (the usual default) to shared color mode (selected using WAVE 's -S command-line option), or vice versa, since WAVE uses different techniques for drawing in the scope window depending on the display mode.
A bug in network file handling can sometimes interfere with correct display of waveforms in the scope window when reading records via HTTP. For now, the only workarounds are to restart WAVE , or to copy the record to local disk files (this can be done easily using xform, snip, or a web browser). Check PhysioNet for updates to WAVE or to the WFDB library that may correct this bug.