Jeep: Inconsistent guage readings in 1977 Cherokee

I am getting some strange readings from my fuel, oil pressure, and
temperature gauges on my 1977 Cherokee Chief as follows;

When I simply turn on the ignition but don'g start the engine, all
three read accurately and stay at those readings. The ammeter, of
course, operates independently of these three, and functions normally
under all conditions.

When I start the engine, they all read accurately at first, then
slowly drop to the lower readings. For example, the fuel gauge drops
almost a quarter of a tank, and the other two drop likewise. At some
point, they may return to normal for a while, especially in cooler
parts of the day (or when cooled by the AC). It makes no difference
if accessories such as the AC are operating.

If I kill the engine, then turn the ignition right back on (or wait a
while- it doesn't matter), they all three return to normal readings
and stay there, regardless of temperature.

Any ideas, anyone? This is something I haven't seen before! Thanks!

Dan

Twice in the last three days, when I started my Jeep to leave work,
the oil pressure gauge stayed on zero (not new-I have a bad connection
on the 4-prong flat plug's orange wire that plugs into the circuit
board <Radio Shack no longer carries such hobbiest parts>). However,
both times the fuel and temperature gauges read accurately until the
oil pressure gauge started working. Then, both of them dropped. Any
possibility that the oil pressure gauge is loose, or something like
that? I'm going to be working on this stuff tomorrow morning, and
would like any other suggestions to check out. Of course, if it isn't
something like that, I'm also going to check out the ammeter, too.
Thanks.

Dan