Unstable wheatstone bridge simulator circuit - help required
You're using R6 in rheostat mode (2 terminal) rather than potentiometer (3 terminal) mode.

While this works in theory, and in circuit simulators, it doesn't work so well with real components. The problem is the current going through the contact resistance of the wiper touching the track. As the wiper moves, it makes microscopic jumps, the contact resistance to the track varies, generating a varying voltage. The output 'crackles' and jumps about noisily, making it very difficult to set a particular output. The purpose of the (seemingly) redundant connection between the end of the track and the wiper is so that when the wiper jumps, the resistance only goes up to the full track resistance, and not open circuit.

In potentiometer mode (below), the wiper carries ideally no current, and stably picks off the varying voltage along the track, assuming a high enough input resistance to your detector.







simulate this circuit &ndash; Schematic created using CircuitLab

The 'opt' resistors are optional. Only include one or more of them in parallel with their respective components if you need to. 

R2opt and R4opt are for coarse balance, to allow you to shift the balance point a little if it's out of range of your pot. The reason to use Mohm resistors in parallel rather than 'a few' ohm resistors in series is that you can leave the basic bridge connected, and dab them on by hand, to see the effect. You can do this with series shimming, but it's not as easy.

R6opt is to reduce the adjustment range, without changing the potentiometer itself. For instance, 100ohms in parallel with the pot will reduce its range by a factor of 2, 10ohms by a factor of 11. Use this to further improve the sensitivity of your adjustment. With it, you can use a larger value of R6 than you might otherwise. Say you only had a 1k pot, then you could use R6opt to reduce its adjustment span.