The oil pressure may still be a mechanical gauge - a pipe from the oil
filter base to the gauge. What sometimes happens is that a couple of
old, old, hard "O" rings inside the oil filter canister allow oil to
bypass the filter erratically which makes the gauge buzz and vibrate at
idle. These "O" rings ought to be replaced at the next oil change; the
"O" rings are available from the dealer. There's no other reason for the
gauge to "bounce around" it should read about 1.5 BAR at hot idle and 3
BAR above, say, 1,200 rpm. BAR = one atmosphere, 14.7 lbs.
Adjust its valves and, while its open, measure the timing chain's
stretch, clean out the "banjo fitting" a fitting and hollow bolt
attached to the very aft end of the intake manifold, replace the two
fuel filters and engine air filter, replace the transmission fluid and
filter. Do these things and it's good for 15K miles when the valves need
an adjustment and another 15K miles for the other items.