Tacho & fuel gauge intermittent fault

Hi I'm new to this site & have a problem with my 1991 Cressida. On hot days the tacho ceases to work & the fuel gauge goes off the top of the scale. Seems to right itself when the weather cools. The other possibly related issue is that normally the fuel gauge does function but shows only 3/4 tank when full. I use the trip meter to estimate my remaining fuel. Is this a common problem? Possibly a loose connection behind the dash? Maybe a dry solder joint on a circuit board? All suggestions considered as I'd like to fix this one myself.
Regards
Peter