Selecting the appropriate sensor

Since your pressure sensor only uses about 6mA according to the datasheet you might be better off with a switched capacitor step up supply like this one: http://www.ti.com/lit/ds/symlink/lm2750.pdf

To get your analog signal down to levels you can read you could just do a simple voltage divider. Or you could use an opamp this paper probably gives you more information than you'd care to know but there's a good circuit at the bottom of it: http://www.ti.com/lit/an/slyt173/slyt173.pdf

Hope that helps.

These problems are hard to diagnose. The first thing I would do is swap the level and pressure ADC inputs to make sure that there isn't some sort of software issue. Even if you've looked at the code a thousand times I would do this.

The impedance of your sensor is pretty high to feed directly to the ADC input. I think you should add a unity gain, non-inverting buffer between the sensor and the ADC input.

Bypass capacitors may be needed. There's not enough information to tell where they should be added or what their values should be. I would be sure to bypass the 5V supply for the sensor. You could add a small capacitor to the output of the sensor as well, assuming that the pressure is not changing rapidly.

You could also try slowing the ADC clock rate to give the sample-and-hold more time to charge. Sometimes putting the processor to sleep during a conversion can reduce noise as well.