With regards to L arm being weaker than R: If you have a gym with overhead mirrors, use them to check your form. Alternately, have your training partner or a spotter watch you carefully to be sure your form is uniform.
Also, keep in mind that a great deal of bodybuilding is psychological. Other things you might do include: Machine bench (support would be a little off doing it with DBs, but you could do that too) one-arm-at-a-time-with-different poundages. You could do (for example):
1) L arm: 80 lbs, 12 reps
2) R arm: 120lbs, 12 reps
3) L arm: 100lbs, 12 reps
4) R arm: 100lbs, 12 reps
5) L arm: 120lbs, 12 reps
6) R arm: 80 lbs, 12 reps
Which is simply ascending sets for one arm and descending for the other. Perhaps your left arm simply doesn't like being compared to your right arm
Just an idea...
-Scott
------------------
-Scott