It sounds like the balance between the LSN and the idle screw is out of whack. This problem is often caused by a too high idle setting being disguised by a too rich LSN. Takes a little tinkering to get it fixed but it's not hard once you have experienced it a time or two and figured out what to do.

Normally just richening the LSN will cure the problem but if it doesn't you may need to try something else. So here is what you do, run it and get it up to temp. Get the HSN set right and get it nice and hot. Come in and let it sit at idle. If the idle stays high for a long time just back off on the idle screw until the idle comes down and you should be set, or at least really close. If the idle is really fast for just a few seconds and then goes right down to a reasonable level then try leaning the LSN.

BUT HERE IS THE TRICK.

Leaning it sounds backward since it is going to make the problem worse but that is the idea. Lean the LSN a bit, go run a good hard lap and come in again. Keep doing this until the idle stays really high for a while (maybe 10 seconds or more) and then reduce the idle screw until you get a low, steady idle.