No governor on that baby---it should be loafing at 85 & 90.  Grade of
gas?  Plugs & wires?  HEI coil, distributor & module?  The long shots:
Knock sensor is signaling the Electronic Spark Control to retard the
spark.

Does it feel like it's leaning out?  Fuel pressure slump?

As these cars age, we hit 'strange' problems like primary ignition
wiring faults and failure to pull enough ignition current.  I'd not
expect that to show up at 85 but it might be worth a look if everything
else checks out.