The regualtor might need replacing, but also the pins where the main plug can become loose and not have contact, or the copper 'strips' can break and lose connection for certain circuits in the cluster. You wont know, until you pull the cluster (have a spare car in case it takes a while to get it back together...).
You might check your 'wiring' under the hood and make sure the temp sensor isn't all gunked up(engine hasn't had a flush in a while), or the temp sensor wire (goes around the front of the valve cover from the 'coil' wiring harness, has a 'thermal/fabric sleeve' on it) may have 'grounded out', broken, or isn't tightly attached.
With the gas tank sending unit, you might also check the wiring leading to the unit. Pull the wire and ground it on the frame and have a buddy see if you gauge reacts. It might be the unit, it might also bee the little 'spring clipped' ground strap that rots out (clips to the 'outgoing' tube on the sending unit, and the main gas line on the body).
-D.Idiot