79 Suburban gauge question

[jimfulco]

'79 Suburban w/factory gauges. All 3 will read lower than usual for a few days, then higher than usual for a few days, then normal again. The oil gauge reads predictably higher when cold & lower when warm, but the range varies. Sometimes it reads 5psi hot/15psi cold, sometimes 35/45, sometimes in between. When running, the voltmeter usually reads noticeably above battery voltage, but sometimes BV is slightly in the red, sometimes just under 13. Alternator is a month old, voltmeter was correct when the old alt. was checked. When fully warmed up, the temp. gauge either reads "almost on cold", or "about 1/3 of a tank". Water pump & thermostat are almost new, coolant is full, & heater works. Engine is high mileage, but I don't know exactly how high.