It only fluctate at idle

I have a 95 bonneville se. On start up it will idle fine and drive fine. Once the thermostat opens up, the Rpm's will fluctuate the check engine light comes on and the car will run extremely lean.But the problem does doesn't always exist it will go from a fluctuating idle at one stop to a consistant ...

If your oil pressure gauge is bouncing up and down in idle first change the oil. Make sure you examine the oil check for coolant also use a magnet to check for metal shavings. This indicates a major problem most of the time anything from a blown oil ring to blown head gasket or possibly a sticking ...

If the terminal are free of corrosion and the alternater is chargin the battery. Check the positve cable going to the starter I've seen the cable hit the exhaust manifold and melt the wire casing cuasing it to arch against the manifold.