You can test your ignition coil:
Chances are the coil will read out of spec and need replacement but there may still be problems elsewhere.
I think what the previous diagnosis found was that coil and the ECM. Both are known to have high failure rates.
Here's a look at some of what your system has on-board (except for the diagnostic scan tool):
If any of these components (except the relays) are out of range from normal, you can get poor performance.
What does the maintenance history look like? Does your Voyager get routine tune-ups, filters, etc?