Sorry what do you mean by "Did you place a cap over the supply lines, directly at the sensor"
Excactly what I asked (cap = capacitor), what puzzles you about it?
I take it that you didn't, so try that.
What model number is the Sharp sensor you used?
How would I go about getting accurate measure from a sharp sensor?
Apart from decoupling the supply lines by the cap, I'd just measure its output.