Reading a vaccum guage

[lukecp]

Awhile back i put a vaccum guage on my truck, and at idle it pulled between 18.5 and 19.5 inches of vaccum. The needle on the guage was vibrating very fast between the 2 marks, but it never went about 19.5" or below 18.5". I have read on vaccum guages in a few books i have and on the net, and they all say that a variation like that is a result of "igniton troubles". What could be the troubles? The truck runs like a top, no misses, hesitation, starts great, ect. My plugs and wires (AC Delco R43S and Taylor Hi-Energy wires) have about 1000 miles on them, but the cap and rotor are over a year old and have 13k miles on them. Any ideas on what would be causing this? Excess play in the distributor shaft? Worn out cap and rotor (they looked OK....)?

Thanks for any help