Ok.... heres what to do.... when you know it starts (on a sunny day),
take a spray bottle of water and while it is running, spray the water over
the wires one at a time.... make sure you get both ends. What you will
listen for is the engine to start bogging or shut off.  ONce that happens,
you have found the source of your trouble and you can replace parts from
there.  90% of the time, the problem will be a distributor cap or one of the
wires leading into the cap is allowing condensation inside causing the
problem you are having.  Its an easy fix and not all that expensive.
   Let us know how you make out...

Fwed