Fiat 124 Spider misfire

[ystephen]

My '77 Fiat Spider runs fine on short trips but if I fill up and drive 100 plus miles, it will begin to misfire and continue to get worse until I stop for a time. I have tried dry gas and it seemed to help, but not always. I have installed a new distributor cap, rotor, plugs and fuel pump, but the problem remains. Any suggestions?

[2ndwind]

Read up on vapor lock and see if this is the issue.
http://en.wikipedia.org/wiki/Vapour_lock

[So Cal Mark]

you mention cap and rotor, what about the points and condensor?

[majicwrench]

Not much to go on. Is possible you have no tank vent. Try removing gas cap when problem occurs.
Is it "missing" which would mean a cylinder not running right, or is it losing power, like all the cylinders not running right??