I started taking ACV + DE ten days ago. I have a history of gout. I haven't had it for 3 years but now I'm having an attack again. Is it because of the orthosilicic acid produced in the ACV + DE mixture? Does orthosilicic acid cause an increase of uric acid levels? I've read that beer contains orthosilicic acid and is good in that respect, but beer is bad for people with gout. Is it because of the orthosilicic acid in beer?

No, it is not the orthosilicic acid.  It is the purines in beer that cause the problem.  Distilled vinegar is considered low in purines.  Raw vinegar though will still contain the yeast used to make the vinegar and yeasts are high purine sources.

On the other hand, is it possible that orthosilicic acid was actually dissolving and pulling out uric acid crystals from my joints, causing a healing crisis that manifests as a gout attack?

No, but it can help repair the damage.

I have stopped taking the ACV + DE. If I decide to try it again, can I avoid having a gout attack by drinking plenty of water so the excess uric acid gets flushed out in the urine?

That may help.  But there are a lot more causes of gout than simply high purine diet or lack of water.  Some other factors include alcohol intake, hypothyroidism, some medications, hemolytic anemia, etc.

By the way, I have been taking Soy Protein Powder, He Shou Wu, and ACV (with no DE) for a long time without any problems. Recently I added DE, Oat Bran Powder, Triphala, and Dulse to my supplements. I also began eating raw mung bean sprouts as part of a salad lately. Do any of these raise uric acid levels to an extent that could be the possible cause of the recurrence of my gout?

Seaweeds, like dulse, can aggravate gout.

Also, what do you think of the connection between sugar/fructose consumption and gout as presented here:

http://www.fourhourworkweek.com/blog/2009/10/05/gout/

Fructose consumption does increase uric acid levels.

I've been fond of fruits, both fresh and dried. I have also been taking Black Strap Molasses, Cinnamon + Honey (2:3 mixture), and a little dark chocolate (72% cacao). Should I avoid these since they all have fructose?

Yes.  Chocolate also raises uric acid due to its xanthines like caffeine.

My foot hurts so bad, it's killing me. I hope you could help!

For starters increase your water intake.  The malic acid in magnesium malate tablets will also dissolve uric acid.  You can also soak your feet in as hot of water as you can stand to help dissolve the crystals.