Oil Pressure Problem

A "Check Gauge" light is on in response to low oil pressure at idle speeds. I'm not sure when this developed, but think it was many months ago, and I suppose I wasn't paying much attention, as the light cleared immediately upon acceleration from idle and never appears otherwise. The engine runs beautifully and apart from this ominous warning, there are no active issues. However, the appearance of the "Check Gauge" light has recently become much more frequent at idle, and the gauge needle at such times is actually in the RED, which I don't recall seeing before, though I might have missed it if it were only briefly in that alarming range.

I've just recently had the pressure checked MECHANICALLY by two different garages. The first said things were OK in spite of the low gauge reading. The second mechanic said he used two different gauges to ensure reproducibility of the readings, which were taken at three different engine speeds. Reading from the second and more particular mechanic are as follows:

1000rpm--04psi (06psi expected)
2000rpm--14psi (18psi expected
4000rpm--22psi (24psi expected)

Signal from oil pressure sender to computer said OK. 2nd mechanic suggested flushing the engine in conjunction with an oil change in hope that it might dislodge suspected crud blocking the proper circulation of oil. It didn't help. He doesn't do major engine overhauls, and discouraged me from opening the engine just yet. I'm a bit nervous, but not enough to have stopped driving this van...