Probably the easiest thing is to go to a wrecking yard and get a couple
of replacement gauges from an instrument cluster.

Replace them with yours and see if the problem persists.

If it does: the fuel gauge may be erroneous due to a failing in tank
measuring device.

There is a module called a temperature compensator.  It may be the same
thing that you called the voltage regulator.  It's a small printed circuit
board about the length of your pinky finger.  It plugs into the back of the
instrument cluster.  It handles voltage to these two adjacent gauges.

If this does not describe what you called the voltage regulator, then I
would suspect this module.  Even if it is, it is easy to get defective
compensator, or to damage it with handling with your fingers exposing it to
oils and contaminants.

I think the brickboard has an article on bypassing this module.  Several
owners have found that it is a weak link.

It seems to me that the old style cluster had a voltage stabilizer on
the back as oppose to the newer style cluster that had a trnsistorized
voltage regulator. On the older units the stabilizer had an adjustable
resistor that could occasionally be adjusted to correct a gigh gauge
reading but the susal answer was just to replace the stabilizer Volvo PN
1234971 $52.17 List.

Bob

If you did not get the gauges fixed yet here is the answer.
Your Volvo has a voltaged stablizer on the back of the instrument cluster.
It is a small rectangular box that has 3 male spade connectors.
Unplug it and install a new stablizer.
The stablizer reduces the voltage to 10 volts for the temp and fuel gauge
only.
The lower voltage is used so the gauges do not vary.
The gauges are 10 volt gauges.
Higher voltage higher readings.
In 1981-1985 volvo switched to a solid state stablizer for  240's.
In 1986 on, Volvo used the comp board.
BD