You need 100 ml stock Xtol per 80 square inches, not 100 ml per 4x5 sheet (20 square inches). Definitely increase dilution to give at least 8 minutes in the developer. I'd even go to 1:3 which should get you over 10 minutes. Longer development times help consistency of the process.
For taming the high contrast you might also want to try stand development at 1:3 or even 1:5 for about 30 minutes. Agitate for the first minute then let it sit for about 10 minutes then agitate again, let sit for the balance then proceed as usual.