I definately agree that you need to use up to 3 fouling shots. If however this does not completely clear up the problem (I'm suspicious of the horizontal pattern) then you may have a problem with the barrel itself having some internal stresses that are showing up as the barrel is heated. This can be corrected with cryo-freezing. See this link:
http://www.diversifiedcryogenics.com/gunbarrels.htm
Here's a quote from that site:
Cryo-Accurizing relieves stress in firearm barrels through deep cryogenic tempering. Stresses cause a barrel to bend or warp as it heats from repeated firing -- warping causes walking, stringing or wandering in the shot group. Deep cryogenic tempering process relieves internal stress in the firearm so the barrel will no longer bend or warp. In addition, your firearm will be easier to clean and give you increased performance, increased accuracy and extended barrel life.
Edit: If I recall correctly, Gander Mountain offers this cryogenic service as well.