Some basic questions.
A. What type of battery? Wet cell? AGM? Gel?
B. Is the battery good, for sure? Some dead batteries show a full voltage, but can't push out any amperage.
C: CHECK ALL OF YOUR GROUND CABLES. Clean the contacts, and tighten the connection. There is a ground cable on the engine itself as well as the battery. This has saved several people many headaches by simply checking the grounds. All the random inconsistencies you're stating sound like bad ground.
In regards to your plugs, you should get the Iridiums in my signature. NGK coppers are great for older Hondas, but were never good for this bike, even when it was new. The new style Iridiums really help the spark, especially with older VN750s.
These are the basics I would check before going crazy on the other stuff.
1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Rebuilt Forks w/ Progressive Springs
VN750.com Grill Cover
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R
1990 Kawasaki Vulcan 750
1998 Honda VFR800 FI
2014 Honda VFR800F
1989 Pontiac Firebird Formula 350