Hi Joe and welcome to RCU,
Sounds like the engine has not reached operating temperature in the first 5 minutes and is tuned too lean for the temp at that point.
I found that the large nitro engines don't reach their operating temperatures in half a tank the long way. Same symptoms as yours.
Expect to warm the engine up in the first tank, then re-fill while running, and start you high speed tuning runs from there.
At this point you should probably richen up the HSN 1/2 to a full turn to make it past the 5 minute mark.
Hope that helps.