Phlipphlop

Just had this symptom on my gal (2004 1.9 TDI) temp gauge reads normal..... drops to zero then after a bit goes to correct temp. After changing the temp sensor in their galaxy, can anyone actually confirm that it was definitely the temp sensor that was at fault? Not an ecu signal fault? If so may look at changing mine, but haven't had this one long so might try and hold off a bit. In addition.... also had problems with my fuel gauge. Read 1/8 of tank when I bought it, put in £30 it went to just under half a tank, used that to 1/4 of a tank then filled it up. It read full however since then after a short while of driving fuel gauge drops to zero and no refuel pump sign on dash. Now always reads full at start of journey, then empty and have now done 130 miles on this tank. Had a gal before (same type/age) and never experienced that, would expect needle to have dropped off full and be above 3/4 somewhere. Could this be a fuel sender problem as well similar to the temp sensor issue or a common earth fault that could affect both? Any help/advice would be greatly appreciated as I've never seen this before. Thanks.