Intermittent readings are usually due to a grounding problem. The sending unit is fed by one wire that has a 12 v supply. The unit itself is just a rheostat that changes the amount of voltage that goes through it based on the position of the rheostat control, in this case, the pressure moves what is esentially a diaphram. Intermittent readings indicate one or both of 2 problems. Either the sending unit diaphram is not moving smoothly versus the pressure on it and/or the wire to the unit is not doing its job. A bad connection at the sending unit is one possibility. A worn wire insulation, say near an exhaust header or equivalent rubbing poinat can also cause the problem.
Chase the wire to make sure it is not grounding out somewhere. If the wire is clean and good, toss the sending unit and try another one. The previous owner's replacement of the sending unit leads one to think the unit is not the problem, the wiring is.