Have you checked the engine oil level? If the engine oil level is allowed to get very low; it can cause this problem; and potentially destroy the engine. If the engine oil is diluted from being used mostly on very short trips; or if it is run for longer than the recommended oil change interval; this can also cause this problem. Mostly short trip driving can require more frequent oil changes than usual. Once every 3,000 miles or 3 months is the rule in those cases.
Using too light an oil viscosity or mixing different oil brands and grades can also cause this. 10W-30 is the most suitable oil viscosity for this age vehicle.
The other thing that can cause this is a defective oil pressure sending unit. This happens occasionally, so if the engine is not knocking when the pressure reads low; it may be worth having a new oil pressure sending unit installed; but if there is knocking from the engine at those times; chances are the oil pressure really is low. Low oil pressure can also be caused by a worn out engine that has lots of miles on it; or an engine which has worn out prematurely because it was not serviced or maintained properly.