Sorry - I did leave out some code.
To simplify things I tried -

DEFINE OSC 20
Goto Start

DEFINE INTHAND HIGHINT

Start:
PAUSEUS 10
Goto Start

HIGHINT: PAUSEUS 10
RESUME

I still get - 'Error[113] c:\Progra~1\picbas~1\incfil~1\pbppi18l.lib : Symbol not previously defined (HIGHINT)'.