Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
Most commonly, this will be a faulty Oil Pressure Switch. Over time, the internal resistance of the switch wears out, and when there is no pressure on the switch (like at a stop) the switch will give faulty readings. Basically- it's just a wear and tear item, and the most likely culprit of your problem. This switch is located on the oil filter housing:
Yes- piece of cake replacement item for a DIY'er. This switch should be readily available at any local parts store, like say NAPA.
Glad I could help! Have a great day!
Isuzu does not provide a spec for oil pressure at idle. They suggest it be checked at 3,000rpm: