I know a number of us have previously discussed focusing screens, and good sources of upgrades and replacements. Following Sean's comments my Buroni wasn't as bright in some lighting conditions as other screens, I ordered a replacement and made the two or three simple steps to replace.
A great deal better, and with the source being particularly reliable and accurate the job was easy and the results pleasing. Details here:
Buroni Focusing Screen Upgrade.
If you operate a focusing screen device, a thoroughly recommended, and easy step. See ya! Skj.