Check the pH of your water first. It may be so high that mixing nutrients per the instructions on your package will throw off your final readings.

If your tap water's pH is lower (lets hope!) then just dilute your nutrient batch with water until it is about 6.2 or 6.3, which is ideal for tomatoes.

If that doesn't work, you might want to consider switching to an "organic" nutrient. There are microbes inside organic nutrient that will somewhat "automatically" maintain your pH.

The real trick will be maintaining an EC of 1.8 to 2.0 without messing up the pH once you have a handle on it!

Try this and get back to us here on the 4m.

During the growing phase plants will absorb mainly nitrates, which will cause the pH to rise. I adjust/compensate the pH by adding nitric acid (5%) once a week.
Once the plants start to fruit the process reverses and the pH will go down. Then I compensate with Potassium Carbonate.

There's a lot of things that need to be determined here.

First off, like they've said already, you need to measure the pH of the water before you add anything to it. Here's what I'd recommend:

1. Fill 2 gallon containers with water.
2. Measure pH of each (should be the same). Record.
3. Mix one container to your standard nutrient strength.
4. Measure pH of nutrient solution. Record.
5. Aerate both containers for 12-24 hours.
6. Measure pH of each container. Record.

Almost always the pH of a given solution (straight tap water even) will change when aerated for several hours. You want to know where that number is headed before you dump it into your system. For instance, if you know that it will start out at 6 pH and drift up to 7, you might want to pH down it a little more to compensate.

Finally, you need to consider the possibility that your system isn't pH neutral. What medium are you using? It's possible that some impurities are getting into your solution simply by it being moved through the system and that's throwing off the pH. If that's the case you may just need to flush the medium, or you may have to replace it.

Whatever the case, we need more information to track down the problem.