Gee, 3 years, nice, but You should consider shooting more
Personally I used tightly capped stock solutions of xtol up to a one year old, and I mix it 1+1 for one time developing.
I believe that weak batches in the past were due to an error in production and quality control, I had one few years ago with freshly mixed solution; it was too weak and I got thin negatives... I always used 5l batches.