As others have suggested, with occasional rough running I would be suspicious of the ignition leads/plugs...at the very least, I would start with new high grade ignition leads (Magnecor have a good rep) and appropriate NKG Iridium plugs.
We had a period of intermittent rough running esp on long hot runs that was only finally quelled by changing out plugs/leads.
If you do/have done this then there is a firm base to go from...
