Varying Idle, 02 Tacoma 2.4L

Curious if anyone can shed any light into what is going on with the idle on my 2002 Tacoma. It's a 2wd, extra cab, 2.4L manual with 135K.

When I start it cold, it idles around 1300. After it gets warmed up, the idle works up to at least 1800. Usually have AC on. When I pull up to a light and push in the clutch, it will briefly level at 1800 rpm, then climb to 2000, drop to 1000, climb back to 2000 etc. It only takes about 2~3 seconds for each jump and drop, and usually does it about 6-8 times before holding steady at 1800 or so.

The refrigerant was low, so I added some, and cleaned out the MAF sensor. I've got a bad downstream O2 sensor that I haven't gotten around to replacing yet, but will shortly.

Any thoughts?

 

Check for vacuum leaks