70 Evinrude problem

[Florida Boater]

1988 Evinrude 70hp. The problem is that at slow speed, like going out a no wake zone, it acts and sounds like one cylinder cutting in and out. Runs ragged at that speed. Idles normal and anything above slow speed it runs good and strong with No missing. I have removed head and installed new head gasket, very good compression readings, I've set linkage and carb timing, cleaned the carbs twice, installed two new sets of plugs, changed coils from known good motor, installed boysen power reeds, and installed new OMC power pack. I have shorted out one cyl at a time when running ragged and two cyls at a time but this is not a problem with one particular cyl. They all reacted the same. I did a ohm test on stator and timer base, checked ok. Everything I've done has not changed symptoms at all. It runs so good at higher speed it makes no sense. I used to be a outboard mechanic but this one has got me frustrared. Any ideas? Possible crankcase leak? Any ideas would be appreciated.