Martin, I usually use mid-grade or premium and I always leave it topped up with premium for winter storage. I just put in some fresh gas as the first thing to do when this problem started.
OK, I put in new plugs, went down to the highway, revved out to 4000 or so in 3 gears and started chanting, "Dave, you the man."
Then I turned around and it started missing again sometimes and sometimes not.
But there's a mystery. This engine has run flawlessly for 8 years. It wasn't even the intended engine but it ran so well it just got left in the car. So I've never done anything to it.
I thought that it'd be good to check the timing. I dug out my very old light and I was surprised that I couldn't pick up the yellow painted mark on the crankshaft pulley.
I put #1 to TDC using a dowel to indicate precise TDC. The #1 rotor position was correct and agreed with the distributor cap. The mark on the crankshaft pulley was way down in about the 4 o'clock position. If I put the timing mark on "0" degrees the piston is nowhere near TDC. Can't for the life of me figure out how this could be. There can't be any change in the relationship between the crankshaft pulley and #1 piston.
I punched a new mark for TDC and advanced the timing a bit. Still a miss about 3500 especially when accelerating in 3rd. 1st and 2nd acceleration now seem pretty good.
I might sub in another coil that I have but it's condition is unkown.
I'm starting to wonder if it's some form of fuel starvation. Maybe I should be changing the fuel filter etc.
This is going to drive me nuts until its solved. I hate intermittent problems.
Ron