My truck has a slight miss! BUt NOT ALL THE TIME?? Help me out here!

[sam's 72]

I have a 305 in the truck, I did a complete tune up on it about 3 months ago, rotor, cap, wires, plugs. All new, it has been running great. Smooth, but before I did this it had a slight miss, but not all the time, and just at idle, not under a load. Well, now that slight miss is back, but it is not all the time. What I mean by this, is some days it runs very very smooth, then one day I might get in it and it pull up to a light, and there it is, it is not a constant miss either, just every now and then you can feel it. Hell, I might go a whole week with out feeling it, then all of a sudden, there it is again, I have also upgraded my chargeing system, I thought it might have been that at one time, but it's still doing it! I have a 140 amp alt, internal regulated. One wire .Sowhat doyou think it could be? I wish I could some how, exssplain better of what it is doing, but it's hard too.
Sam