Hi
I've just checked out that brush (in Kyle Websters Megapack Drawing Box - thanks to Jane-E for finding it). The problem is indeed the texture. It is based on a 2500 x 2500 pixel texture that does not tile well as you can see below where I've used it to fill a 4096 x 4096 pixel area

It can be fixed.
1. In the brush panel click here to save the pattern :

2. Open a new document 2500px x 2500 px and use Edit >Fill >Pattern to fill it with that pattern
3. Use Filter >Other Offset and set the offset to 1250 in each direction
4. Use the spot healing brush/clone tool to cover the now visible join
5. Use Filter >Offset again to restore the position to the original
6 Select All
7 Edit > Define Pattern
8 Go to the brush panel and select the brush
9. In texture go to the top and select the newly saved pattern

10. Click the drop down menu at the top right of the brush panel and choose New Brush Preset.
11. Give the brush a new name and save it
Dave