2002 Ford Focus 1.8 5-door Ghia?

The oil pressure light on the instrument cluster of my 2002 1.8 5-door Ford Focus Ghia is intermittently flickering on and off after an early morning cold start. After the engine bay has begun to warm the problem ceases. The car is garaged over night. The problem appears to be greater during days of high humidity. This is not an oil pressure problem, the engine runs beautifully and has only done 14,000 miles from new with oil and filter changes twice a year regardless of mileage. I have owned this car from new.