Basically everything you said and you in the response is correct. I have the same problem as you, I get stronger and make gains but still have that dead achy pain deep in my arm... I feel like its bum. its my left arm, and even though it feels weak it always out preforms my right in bench, curls, etc. its quite odd. like he said, locking the hands in one position is bad for all your joints, and free flowing IE dumbells is always better for the joints... lack of forearm work is same as lack of stabilizers, using a barbell to your joints is the same as using certain curls to your forearms. Think, if you used a smith machine to do all your squats then went to free bar, you wouldnt be able to put out the same weight because of lack of stabilizer muscles. same with curls you isolate with the preacher to get bicepts strong and then other exercises hurt your forearms because they arent up to par. try doing more pull up, its a great bicept exercise and the many variations is better than just one type of curl, which is underhanded. mine have actually seemed to go away but do come back if I work bis too hard. Ive found preachers still cause this pain for me, and you can notice that your elbows are forced outwards during the contraction. This is because all the pressure aka the weight your moving is pushing on your forearms and joints, and the natural rotation has to support all that weight. weight is moved by muscles which are connected to tendons and ligaments and such which are connected to the skeletal structure supporting the weight. so when the motion of curling occurs, all the weight is being forces through those tendons, and magnified during the awkward motions that your forearms and elbows go through while making the motion. this puts alot more strain on those tendons in the forearm than they are made for, causing the stretching and pain in that area. this is the tendonitis (sp?) so pretty much, certain exercises cause strain. Find out which ones cause the pain, and which ones dont. Do the ones that dont. If they all hurt, stick with rows and pull ups. my bis grew 2.5 inches doing just those and very little curls. and there are plenty of other lifters who will tell you the same, that pullups plus your back exercises will give your bis plenty of work.
Anyways if anyone else has anymore information about this, go ahead and post. lets try and make this a good source of information, as this is a fairly common problem as I have heard.