Oil pressure warning light coming on!

[kkmchan]

Bought my 2000 Boxster 6 months ago with almost 90k miles on it, had a PPI done at AAA told everything good. Symptoms: After driving awhile, usually when slowing to a stop in low RPM situations, oil pressure warning light comes on. Notice some high pitch sound from the engine. Gas it up and the light goes away, also seems to go away when at idle. No leak on the floor and dipstick showed good oil level. Look from top of engine compartment no obvious leak.
I've searched and read many posts, suspect oil separator vs oil filler tube vs oil pump problem? Can you guys shed some light on this? Oil separator may be a DIY but I'm not so sure about oil pump...
Thanks a million and happy Father's Day to you all!