Motorcycle Repair Questions? Ask a Mechanic ASAP
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you have covered all the usual suspects.
verify your breathers are not restricted.
at this point I would also recommend dropping the oil pan and checking for an obstruction at the feed port and also at the feed inlet to the engine, and the passage between the inlet and the oil pump/cam plate.
it sounds like the feed is obstructed or air bound.
in fact....the first thing I would do is pressurize the oil pan through the dipstick hole while cranking the engine and see if your problem doesn't go away. use no more than about 5 psi. wrap a rag around a compressor nozzle to get a "tight-ish" fit in the hole. most tech's have a dipstick cap with a hole drilled in it just for this purpose.