Why am I getting a lot of bubbles coming up from sink when it starts draining? After the bubbles stop, then it just drains very slowly. I have already cleaned out all of the debris from the pipes underneath.

You have a blockage or partial blockage in the line, probably before the vent pipe. What kind of sink, kitchen or bath?

I agree that you have bockage but it could be a blockage in your vent stck. Can you get on your roof safely?

I agree that you have bockage but it could be a blockage in your vent stck. Can you get on your roof safely?

I can definitely get on my roof once the weather warms up a bit.
BTW - I have 2 vent stacks... smaller and bigger one. Should I assume 1 is for the basement plumbing and the other for the upstairs/main floor? And IF that is a certainty, wouldn't then all the sinks, tubs etc, on that same 'line' also have the same problem if it was the stack??

Why not just remove the trap and arm and snake from the sink drain?

A vent stack can be blocked above the sink drain.

Yes, but I don't think that would cause air bubbles. That would cause traps to siphon dry. Blockage has to be before vent.