So my 4 banger has a rough loping idle. The tach needle constantly quivers when the engine is cold, but smooths out upon warm up. When she's hot it will lope every few seconds. I've put a vacuum gauge to it and the reading was consistent with a leaking manifold/valve seal. It fluctuated quickly between 15-20 mmHg. When I changed the spark plugs, plug #3 had deposits on it which matched up with a leaking valve seal on several charts I found.
Any input would be greatly appreciated.