Found that my fuel sender gauge sensor was malfunctioning -- should give 110 ohms at empty, 4 at full (found somewhere here on Mud). My readings were all around the place. The problem lies in the way the resistivity is generated; thankfully one can remove the metal housing of the rheostat (which also keeps the fuel level warning sensor). The wiper of the rheostat is connected through bronze/brass bushings, and conductivity is supposed to go through there. Mine was way too loose, so sometimes there was and sometimes there was no continuity.
I sanded both the wiper and the wire with 400 grit (reaaaally softly, the wire is very thin at parts). Helped a bit, but the hinge seemed like pressed or riveted, impossible to repair. That I overcame by soldering a wire directly on the wiper surface and leading it out through the previous warning light connection point. To preserve factory connection, it would have to be connected to the body of the sensor (it is probably connected to ground from factory). I don't use that anymore and my warning lights block is still in the design process, so I'll solve that later.
Works almost 100% again. There are points and angles when the wiper detaches itself from the wire completely (the hinge is really wobbly). After some testing I'll decide if the whole assembly has to be drilled out and replaced with something more snug..