An unadorned HyperCard 1.2.5 stack should work fine. Your stack may have
an XCMD or XFCN that is interfering with the altered QuickDraw command set
that the Macintosh Display Card 8*24 GC uses. As externals are processed
by HyperCard when a stack opens, any incompatibility (not bugs, though)
shows up.
We recommend that users of the Macintosh Display Card 8*24 GC upgrade to
HyperCard 2.0, mainly for the sake of performance. HyperCard 2.0 uses true
QuickDraw commands, which the Display Card 8*24 GC accelerates. (Earlier
versions of HyperCard bypassed QuickDraw entirely.) Older stacks should
work, but performance will not improve.