GZ250 Surging & Backfiring

[GreenFamily]

I just bought a 2005 GZ250 with about 16k miles. It was surging at about 3/4 throttle so the dealer flushed the carb for me, it didn't change anything. The mechanic suggested running a tank of fuel with Berrymans B12 or Seafoam to see if it improves. I ran the tank down to about 1/4, put about 6oz of B12 and filled it up with 89. I've put about 85-90 miles on it since then it started backfiring about 15-20 miles ago. Also it died on me twice this morning at about 45mph during my 25 mile drive to work, had to pull over to restart it. It fires up easily and idles great.
Might be work mentioning that I didn't realize I had it in PRI for about the last 100 miles but it's on run now.