I'm having similar problems. Especially when I look at photos from previous contests - my arms/delts/chest = front torso lags way behind my back and legs.
After years of pretty rigorous heavy ez-curl bar work - the excessive pain in my elbows and wrists was just not worth it = and my biceps weren't improving. In fact, even some machines - especially when focusing on partials at the low positions stress my elbows and wrists such that there sore to the touch. Occassionally, when I focus on partials from the top position - my shoulders seem to give me problems.
Solution - is cables - b/c of the constant tension they provide and being able to position your elbows, hands, and joints more comfortably. Some machines still do the trick without noticeable pain.
With the sled I've found I actually prefer almost a gironda style curl - elbows very tight to sides - not in front of the body and high - and performing partial movements. This seems to minimizes back and lat involvement. Then I'll often add a backwards drag by putting my arms in the top position of a bicep curls and pulling, sometimes using partial top reps and mid-positions holds....seems to be helping...
And yes, I think addidng the eccentricless bicep work every day if you can manage it will help tremendously.