1989 240 DL with 316+K mi. Car has suddenly started to run rough. Starts right up, idles very high then settles to lower RPM after about 15 secs. After engine is warm (blows heat into interior) the RPM is so low it sounds like the engine is about to stall, but it doesn't. It just idles very rough and the engine "shakes" on its mounts as it idles. So, I cleaned the air hose from the air filter to the throttle body; cleaned throttle body, flame trap and associated tubes. Found no leaking vacuum lines. Pulled the 4 spark plugs & found normal wear. Took car for short test drive after cleaning. With warm engine, got smooth accelleration to about 50 mph, then "check engine" light came on, so I returned home. Pulled code 2-3-1 "fuel system compensating for rich or lean mixture at cruise". Haynes manual states this code will not make the check engine light come on. I cleared the code. Next day I ran the car. The check engine light was not "on". Took car for test drive with same behavior as described above. Idles rough, accelerates smoothly & runs smooth at higher speeds. Check engine light did not come on. Checked and found no codes registered. However, I hear a slapping sound from under the valve cover that I have not noticed before this "rough" episode. Local shop said they'd hook it up for diagnostics for $90, which they'll credit to parts & labor for any repairs I authorize them to do. What else can I check, or should I get out my check book? Appreciate any thoughts or suggestions. Thanks.