Vinegar is not great at controlling PH. It doesnt last long and breaks down. Its also an organic product, and can feed/speed up bacterial growth.
Use diluted Phosphoric acid (most commercial PH Down) or sulfuric acid (regular battery acid). Sulfuric is by far the cheapest option. By the way, sulfuric is what most large commercial growers use around the world, so its perfectly safe.
PH swings can be cause by:
1) temperature swings in the water. Colder water holds more C02, which when disolved in water becomes carbonic acid = lower PH. Warm water holds less c02 = less carbonic acid = higher PH. In my tests, with my water, 10 deg F change can cause 0.1 to 0.3 swing. Your results may be higher or lower depending on your water and nutes buffering capacity.
2) Bacteria/algae/slime etc all drive PH up as they feed. More bad crap = faster PH rise.
3) Roots feeding unequally - can drive it either way. the unequal feeding leaves some nutes low and other hi and it gets worse over time as you re-circulate the water and the imbalance grows. the imbalance can be caused by wrong EC levels, or out of range PH, or sick roots. PH is usually the culprit.
Check your roots, rez, etc for smell, slime, cloudy water, discoloration, etc.
By the way, roots do not turn brown from nutes. They turn brown when sick or when dying.
