Among the scattered few Starlin issues for Marvel in that period I never purchased and don't have are
DOCTOR STRANGE 23 (June 1977), 24 (Aug 1977), 25 (Oct 1977) and 26 (Dec 1977).
And
AMAZING SPIDERMAN 187 (Dec 1978).
And
RAMPAGING HULK magazine 4 (Aug 1977), which I do have.
And a few painted covers for
MARVEL PREVIEW and other Marvel magazines.
If there are any other Marvel stories by Starlin in that period, I'm not aware of them.