I think i'd grab the bull by the horns and change the oil pressure switch for anew one Jack, just to eliminate it as a possible cause.
It could be something like the oil pressure relief valve spring is a bit weak with age so is a bit eager to open the relief valve or possibly there's a small bit of grit/dirt in there and it's causing a slight bypass effect on the relief valve so that oil pressure is slightly slow to build.
If the "timed lights" theory doesn't show up as being the culprit, i'd give another switch a try first, followed by a good engine flush at the next oil and filter change, meanwhile add about 0.5L of synthetic ATF to the engine oil to help clean things through and will also give your hydraulic tappets a bit of a boost.
