Mine was doing the same thing then I had the engine checked for oil leaks they found some and that I had a blown intake manifold gasket. Now I don't recall any coolant leakage via a puddle or smell and I don't remember the reservoir ever going low. But after they replaced it the oil gauge stopped jumping all over.
The oil leaks came from the oil pan gasket and valve cover gasket, I don't think this affected the gauge since I've not seen it affect the oil pressure gauge in the past with other cars except to fall to 0
when all the oil is gone.
Correct me if I'm wrong.