1999 Honda Accord

Hi, first thanks for helping. Own a 1999 Honda Accord lx 4Cyl vtec 4 door. The problem I have is with the motor at iddle speed, the car makes a noise very similar to an old volkswagen beetle and has a rough idle. The car runs well at highway speed, smooth and quiet but at idle the rpms are very low very close to turn off, at traffic lights I have to put it in Neutral and keep the rpms up. The problem is worst when the air conditioning system is on. Sometimes it is hard to start. I checked the compression on all cylinders and is the same, old plugs llok good, no water or oil, new plugs have been installed. It is also very slugish and sometimes it hesitates under acceleration. My local mechanic says that he cannot find the problem. An engine light has come on and when is read by the scanner he has no usefull information. The timming belt was replaced, but the problem continues.
From reading this site the problem could be the distributor, how can I verify that it needs to be replaced withouth buying a new one. What sensors could be causing this situation.