The phrase "rather unstable" is correct and usable in written English.
You would typically use this phrase to describe something that is uncertain, unpredictable, or that changes often. For example, you could say: "The stock market has been rather unstable lately, so I'm hesitant to invest money."
"The Chinese economy is looking rather unstable," Mr. Kim said.
Gender is a rather unstable concept in Indian art.
Malonic acid itself is rather unstable and has few applications.
Most of these people were highly intelligent, rather unstable, and good at getting by.
The girl inside the letter proves a rather unstable arrangement and eventually the whole number is canned.
Ethnographies were not dispassionate records of cultural facts but rather unstable "fictions," shot through with ideology and observer bias.
I find the state of being in Palestine very similar to experimental sound production, as the latter is not independent, but rather unstable, broken, volatile, disturbing, and quite cacophonous, not unlike the sounds of war".
Because of the comparatively low densities and temperatures, the chemistry is very exotic, as judged by terrestrial experiments; some rather unstable species can exist in space because there is not enough energy to convert them to more-stable forms.
Faster increases in prices foster the belief that future increases will also be stronger, so that higher prices fuel demand rather than dampen it.The need to explain so much of Britain's house-price inflation by a change in expectations, writes Mr Miles, "suggests that the current level of house prices may be rather unstable".
Despite being the code of life, DNA is rather unstable.
For instance, native soybeans make an easily oxidized and rather unstable oil.
