All those big loops of wire are going to act like antennas. A microcontroller is a noisy thing. And your instability is very small.............Plugging and unplugging wires from an Arduino without powering it off first is a risky thing to do.
First off those capacitors are not ceramic ones
Next all four sensors are in the same liquid, can you do that?
You might have burned the sensors by messing about with the wiring when the power is on
For example, if i don't connect the BNC connector, i can get the results anyway instead of saying "Check the Probe".
Is the temp sensor dipped in the same liquid as the ph sensor