Have you read this article by Patrick Gainer at unblinkingeye.com? Neat the end of the article he shows different variations of the developer in two tables.
Your formula is not the same as the formula Gainer describes in the article. I think that the main cause of the problem is too much phenidone in the developer. How much dry phenidone are you actually dissolving into a liter of developer? If you are indeed using .1 grams of per liter of developer, then you have 5 times the required amount in your developer. The correct amount is closer to .02 grams per liter. Gainer added 2.5 ml of a phenidone solution to a liter of developer. He made this solution by dissolving .65 gram of phenidone in 80 ml of 90% isopropyl alcohol or denatured alcohol.
You also have 2 times the required amount of ascorbic acid in the developer. The amount of sodium carbonate anhydrous should be 5 gams per liter. I think that the excess phenidone and ascorbic acid are causing all the trouble.
I think you would have better results if you mixed the formula as follows;
Water 20C 750ml
4 g/l sodium carbonate anhydrous Keep this the same for now, since the quantities of ascorbic acid and phenidone will be reduced.
2 g/l ascorbic acid (pure)
2.5 ml phenidone solution
Water to make 1000ml
You may find that the potassium bromide is not needed.
I hope this helps, Post back if you try this, and let us know what the results are.