YANMAR 4jh5e low idle problem

[CaptainRonKin]

I am having a problem with low idle on my 4jh5e YANMAR. The engine starts up fine and runs fine. I keep it at 800rpm for idle. Again it runs fine When it gets up to proper temp and I run normally between 1800 and 2000 rpm but when I bring it back down to a low idle it cuts off. It will level off at about 800rpm and then just dies. It will start right back up again and then do the same when I bring it back down to low idle. I have tried everything I can think of cleaned injectors, new racor filters, new fuel lines, new water separator filter, new electric fuel pump, I replaced the entire fuel injection unit new which was very expensive on advise of a mechanic, checked for blockages, cleaned the mixing elbow thinking maybe a back pressure problem. All have not corrected the problem. I have 1500 hrs on the engine. Would appreciate any insight or has anyone else had this problem. Thanks

[DeepFrz]

Have you verified that the engine is actually idling at the indicated RPM?

[CaptainRonKin]

I have not verified the rpm I will do that. I have adjusted my idle from 700rpm to 800rpm and once at 900rpm Thinking it was too low, with no difference it will still cut out once I come down to my low idle.