Calling PBP library routines from ASM interrupt handlers, is generally not a good idea.

The commands will overwrite PBP's system variables in the middle of working on another command in the main program. The effects are completely unpredictable.

In your case, the only system var used by PAUSEUSL is R0. So if you save and restore the R0 Word variable much like psave and ssave, it should work better.
<br>