I agree with going to a flat base bullet. The 2 groove barrel in my No4 Enfield does the same thing with boatails.
The original '06 load is 150gr bullet @ 2700fps.
I would load up some 150gr flat base bullets and keep the velocity down some.
Remember it's a 26" barrel so you can load it down a little from the 24" barrel loads to achieve the same velocity. - Bill