I am having some intermittent idle issues on my accord that come very close to stalling, but it has never stalled on me as of yet. Generally the engine runs very smooth with no power issues and no missing, including at idle, but every now and then it will idle low at about 600 rpm (according to the dash gauge), but even at that low of an idle it still feel fairly smooth and is not shaking or feeling like it will still...so that is one issue, since it seems that it should be idling in the 750 rpm range.

The larger issue is that sometimes the engine will nearly stall, with some shaking, but it never actually stalls. When it does this it will typically dip down to around 400-500 rpm and then come back up on its own, or I'll rev the engine to make sure it does not stall. In nearly all cases, it only does this when I come to a fairly sudden stop, like in traffic or when coming up to a red light. Not as severe as slamming on the brakes, but just a somewhat quick stop as opposed to a regular/slow stop.

Today in traffic when I came to a quick stop on the highway, the RPM dipped all the way down to what appeared to be 100 rpm at the very bottom of the dial. The engine shook as expected, but I pushed the throttle and kept it from stalling.

What are some possible causes for this, and what tests can I do to diagnose this? Thank you!