you might richen the choke on the chance that it has a slight vac leak. As said on the ballast when it starts to act up you could bypass it with a jumper with alligator clips on each end to ans that ASAP (gotta do it fast when it is hot). Might check Rotor Phasing (my baby) as it might be borderline & then cause the problem when the system gets hot (leaner & higher resistances). Keep us posted!