the fuel guages on the 88 on volvo 740s are renowned for being dodgy. I put up with it for a long time just putting in £10 worth and watching the trip mileometer ('m in the UK). I suspect it's down to the three pins on the back of the fuel guage not making proper contact with the small plug that contacts with it off the printed circuit at the back of the clocks. Haynes manual does give a way of checking the guage by disconnecting near the pump sensor and putting resistors of a certain value in circuit with it to test the guage. I replaced the guage, it read for a while then packed up. i gave it up as a bad job, left it meaning to deal with it again, then bought an 850. Try tightening up those pins and let me know how you get on. - Roy