That sounds like the temperature compensating board problem common in that vintage 240. I don't know if the 740 uses that compensating board. If so, you can clean the board's coneections, resolder the board and you might have fixed it. The voltage regulator is usually on the back of the instrument cluster and is small (1/2"), three lead, black, held by a screw through a metal heat sink tab opposite the three leads. You could clean the leads and tab while your're in there. NOTE: The 740 may not match the 240 in all this but I doubt that Volvo would be using totally different parts in the instrument cluster of these two cars. Maybe the 740 experts will tell us more.