[v8volvo]

Been a while since I have had one of those clusters apart but to my recollection there is a little voltage supply unit somewhere on the back of the cluster that feeds power to those two gauges, since IIRC they run at less than 12V. Maybe if that unit were failing or if it had a crack in a cold solder joint, it could cause the gauges to temporarily quit working?

When the fuel and temp gauges drop to 0, does the speedometer and tach and warning lights still work fine? In other words, it's just the fuel and temp gauges that seem to be having a problem?

If you slap the dash or somehow shock the cluster, does it ever get the gauges working again, like there's a loose connection that is getting jostled?

I'm pretty sure the whole cluster gets power from one source and then distributes it to the different gauges so don't think it could be a fuse or circuit problem but can double check on that. Think I have the greenbook diagram somewhere for the cluster.

One thing you might try would be swapping in a known good different cluster temporarily, to see if the issue disappears with a different cluster. I think I have a spare one for an '84, worst case scenario I'd be happy to send it to you and you could give it a try.