BAsque1 First name would be nice for free info.
There is no 100% way to check the cap. How does the center contact look? MOst times these things burn out the carbon center with high voltage coils. You should always replace both at the same time for maintainance. Have seen a couple of caps lose their cnenter point and still fire off the plugs but run ugly.
Coil you didn't say which coil from MSD you are running, Some
oil filled coils MUST be mounted vertical and not horizontal. If you can also mount it off the motor this helps. Unless it's an "E" coil, vibration and overheating 1/2 the coil windings, you can get total failure or weak spark to the plugs.
There is a basic test for the module inside the distributor. I have only seen 1 module go bad over the years.
The last thing is the 6AL module. Yes they fail. MSD will rebuild it if needed.
Start with the cap and road test. If still doing this, you choice is coil or module. If it's 1 missfire, make sure the pinwheel inside the distributor is clean, blow out with low air pressure.
If there is no change, start with pulling spark plugs and look for a problem or different color than the rest. Check the valve lash at this time for all cylinders. Write back and tell us what you have. Good luck Rick L. Ps LIke others have said with MSD buy all the parts and install in a bag in the trunk for the ignition system. Coil, msd module, spare distributor, ballist resistor, and 1 long spark plug wire. Basic tools to change out any one of these on the road. Been there done this on the track. Some of the guys use a standard black cap on there MSD system and have no or little problems. Look up old threads about running other caps on the MSD. THey have solid centers and brass contacts like the old accell caps had.