My gas gauge used to act jumpy, particularly around the 1/4-1/2 tank marks. It would read a higher 1/2 tank left and then suddenly drop to the correct 1/4 tank full. My habit at that time was to keep a relatively low tank level, i.e. fill up $20 dollars worth when empty. Searching the boards indicated the sender unit might be faulty. Some people replaced the unit only to have the condition continue. I have since kept the tank between 1/4 to full and the problem has gone away. But since your coolant gauge also acts up, it sounds like something electrical.

glenmore
1991 300CE
1990 LS400