Over 900 ppm of sulphates in your water is really high. My city water report says that sulphates of less than 500 ppm are desired for aesthetic reasons.
According to Noonan, sulphate levels above 500 ppm lend a harsh bitterness. Levels of 150 ppm or less are preferable for most beers, unless they are highly hopped. Palmer states that levels greater than 400 ppm are harsh and unpleasant, and that ideal levels should be between 50 and 150 ppm.
Unfortunately, I don't think sulphates are easy to remove from your water. You might consider buying spring water, or using demineralized water + salts.
I just noticed that on page 157 of Palmer's 3rd edition of How to Brew, he actually states that sulphate levels over 750 can cause diarrhea. You might want to check with your city water supply technicians to confirm that 978 ppm figure, or ask why those sulphates are so high.