85 380SE intermittently looses power and blows black smoke out tailpipe

[bcolins]

My 85 380 SE recently started occasionally losing power and blowing a black cloud out the tail pipe. If I just drive with light throttle for a minute or put the trans in neutral and rev it a few times,......or sometimes just downshift or even.....just floor it..., power comes back and the black cloud out the tailpipe goes away,...........for a while,.......keeps coming and going while I drive.

Clearly is a mixture issue,.......that changes from time to time as I drive.

Anyone experienced this? Suggestions on what I can look at?

could I have a vacuum leak that is coming and going that would somehow cause this?

Brian