First off: I usually mix my fixer with steaming hot water and don't remember having ever seen Sulfur precipitate out. Even if I did see Sulfur (from lowering pH too much) it would recombine with Sulfite to form Thiosulfate again as soon as pH went back above 4. Since you seem to have difficulties acquiring photo chemistry (which is not a surprise given your location), let me list some tests you could do, some of which should help you find the culprit and possibly a cure:
- If you mix D76 and your milky fixer in 1:1 ratio, any Sulfur precipitate should quickly disappear. D76 both raises pH to a point where Sulfur and Sulfite form Thiosulfate, and it supplies ample Sulfite for this purpose. Please report if the precipitate does not dissolve within a few minutes. The next tests serve no purpose if that mix still shows a precipitate. Use just enough liquid so you can positively see the precipitate (or lack thereof) as the mix won't be useful after the test.
- If you slowly add Baking Soda (start with 1g/l and go up to 20 g/l) to milky fixer until it stops bubbling, and the precipitate disappears, you have solved the problem. Use the same method to restore your main batch of fixer.
- If D76 restores your fixer but Baking Soda does not, you could try adding Sodium Sulfite to your fixer. If you are unable to source Sulfite directly, you can try using Hypoclearing Agent for this purpose. Start with 2 g/l and go up to 10 g/l and see whether that fixes your issues.