 Dirty Air Filter |
 Clean Air Filter |
Because the filter was dirty and clogged, the engine was runs "rich". It would explain the black smoke and black spark plugs if you have them. It also explains that the rpms only bounce around when at idle, because when driving, a little more air is getting through the filter so it was fine.
Lets clean it!
First rinse it out good and throughly with fresh water. Then take a good cleaning agent such as K&N cleaner and spray it on. Let it sit for a few moments, then rinse it out. Keep doing this until no dirty water is coming out of the filter. Make sure the filter is completely dry before putting it back onto the intake!
Other things you can do:
Check/replace the vacuum lines then i bleed the coolant. Remove the radiator cap and open the bleeder valve on the corner of the head. Fill with coolant until coolant is POURING out of the valve. Shut valve while coolant is still rolling out. Top off radiator and put cap on. IF it STILL bounces, jack the front of the car up about 6-12" and repeat the above.