while it may seem like a silly question, have you checked the oil level? obviously an extremely low oil level can cause low pressure, but so can overfilling the engine (the crankshaft can 'whip' the oil to create lots of air bubbles which in turn lowers the pressure). otherwise, i'd look into checking the VVT and VVTL oil control valve filters (see KenshinX's signature) and go from there. if all those are fine, in addition to the things you've already done, it could be the oil pressure sensor itself as already mentioned.