Fuel and Temp gauge

These two gauges are driving me nuts! For about 10 years the temp and fuel gauge on my '72 Valiant have not worked. I'm not a skilled mechanic by any means but I've attempted to fix them every now and again with advice from people on this forum and people I know. I've replaced the gauges, replaced the cluster voltage regulator, tested the blue wire with the white tracer. Nothing has fixed it.

Then all of a sudden last night for no apparent reason both gauges show signs of life and start moving! They both seems to be off with their accuracy. I have a full tank but the fuel gauge only went up to a quarter of a tank. Does any one know why these gauges would spring to life? I had replaced the brake light and dome light fuse a few days ago but that's it. I'm excited that they are showing signs of life but I'd like to know why and maybe then be able to make them show an accurate reading.