the first thought that came to mind when I saw the title was FPR, but you replaced that...
if it does it when it the the engine is cold, it's probably not the o2 sensor.
you won't ever see a check engine light illuminate, '87 doesn't have a functioning CEL
; which will complicate thing if you don't have a reader. The CEL
is burnt out in my 93 Ford, so I tape a light up to the windshield and wire up the diagnostic connector to pull codes (I'll get around to replacing it someday
). Can use an analog meter too, but easier to have that light in your face in the cab. But you should definitely pull codes.