Some more info would help people make better suggestions for your situation.

What type of water are you using? Some city water and some well water is very hard and can cause some trouble with mineral build up and ph in hydro sysytems.

What type of media are you using? Certain types of media need to be pre treated or soaked before use or you can have ph troubles. Gravel I think needs to be treated with superphosphate to deal with the lime. Rockwool might need to be soaked with pH adjusted solution before use.

What type of nutrients are you using?
How are you adjusting your pH? organic pH adjustment might work in the short term but tends to break down quickly causing wide fluctuations, example vinegar.

What type of system is it?

You mention your solution sours, what does that mean? I always thought sour meant acid but you are having high ph so I'm confused. I wonder if perhaps you are not getting enough airation and mixing of the nutrient. Nutrient that sits stagnant is not a good thing. An aquarium bubbler is an inexpensive fix for an indoor system and in some situations will work for outdoor systems too. One way to stir up or airate a large outdoor tank is to use a larger pump than necessary and add a bypass (like a y adapter for a hose so you can adjust the amount of flow to each side) to the outlet so that the excess water being pumped can be used to stir the nutrient. If you already have a pump that is just the right size for the system then a second small pump that can run all the time or an air bubbler can be added to the tank.

Hope this helps