1995 Pontiac Transport Surging

My vehicle has been surging while driving for some time now (over a year). I have replaced the MAF sensor. When I manually check the code, it tells me that it is the O2 sensor. I have 2 (1 upstream and 1 downstream) but doesn't specify which one, so if I replace the O2 sensor, I will have to buy both, so I want to make sure this is the issue.

What it is doing is while driving (not idling), the engine will surge. If I let off the accelerator, it stops or if I accerlerate more, it will stop. It use to do it only at around highway speeds. Now it does it at anything above lets say 30 mph. So it is getting worse. It almost feels as if the air/fuel mixture is wrong or the timing is off.

And since I am asking this question, I might as well slip this one in which I have no ideal what is wrong, but once again has been doing this for just as long (hoping they aren't related). I hear a swishing sound coming from somewhere around the heater core. Does this mean my heater core is bad and needs to be replaced? The heater won't work at times and there is brown residue in the coolant (which I have flushed a few times but it still comes back). WHat could this be?