The coil is triggered by the igniter which is controlled by the ECU. The ECU knows when to fire based on signals from the distributor's cam position sensor.
Check the wiring to those three components and test them per the TSRM. It's possible it's the igniter.
Also check to make sure the block is well grounded. From the factory the small ground line on the exhaust side may have been sufficient, but as the wire ages it goes bad fast.
MK3 TSRM On-Line