After replacing the water pipe, from water pump to thermostat, and having much of the intake pipes off, the car has been having an erratic low idle. Dips to 400 or 500 then surges up above 1,000 then drops back down to normal. Only notice it at idle and have checked the hoses, clamps, even did a smoke test. Nothing obvious and it seems like it could be a myriad of things but thought I'd ask here.

-valve cover?
-pcv valve?
-throttle position sensor?

 

all of the above PLUS the PCV pipe that routes thought the intake. there is also the gas tank purge valve.