What's a good supplier for the Norvel .25's?
IMHO, the easiest source of obtaining a Norvel 25 is by whipping Sheepy's and Mr.Clean's butts in rc combat at the
2001 Misery In Missouri combat meet.....which is how I got mine.
The second easiest is ordering one from
http://www.waverc.com 
rhd,
I have my PBFs balanced at 5.75" to 6" back, but I recommend a first time PBF pilot start out at 5.5" back. At the 5.5" CG setting, the elevons will be trimmed pretty flat with little or no upward reflex. Then use your trim tabs as necessary to get your PBF flying fairly straight. The farther forward your CG is, the more upward reflexed elevon trim you will need.
But no matter where the CG is set at, the PBF isn't really very stable, and therefore always requires constant attention on the sticks. You can trim the PBF for one particular throttle setting at one particular airspeed, but if you change either of those, the PBF will need to be retrimmed in order to fly hands off again. Generally I trim my PBFs to fly mostly hands off in cruising flight, with the nose aimed up at about a 20 degree angle.
As far as why your PBF used to loop straight, and now since you changed the elevon trim it doesn't loop straight.......that by itself shouldn't have made any difference.
Did you change the CG?
Are you using a different prop than before?
Are your elevons are no longer tracking with each other throughout the entire range of motion?
Did you add a leading edge stiffener?
Did you rearrange the radio that might have altered the airflow pattern across the wing?
Did you alter your muffler setup which might have altered the airflow pattern across the wing?
Did your wing develop a severe warp from sitting or crashing?