Aero position: Pressure on forearms

I don't think there is any way that you're getting a buildup of lactic acid in your arms.  My guess is that the problem is due more to how your arm position is impacting nerves in your arm and it results in a numbing sensation (similar to how some people's hands go numb riding in a road position).

Contrary to what someone posted above, you don't want your core doing a lot of work trying to maintain an aero position.  If your core is working hard, then you're probably too stretched out.  Furthermore, you DO want weight being supported by your arms.  HOWEVER, you don't want muscles to be doing the work - you want your skeletal system supporting the weight.  You achieve that by trying to get the 90 degree bend at your elbow.  When I'm in an aero position (I have a road bike with clipons) I have a 90 degree bend, my bones are supporting the weight, and there is very little tension in my hands.  In other words, I don't have to grip the aerobars with a lot of pressure to maintain my position.  You may have to play around with the pad positioning to get the right feel for your arms so that you don't get the numbing sensation. 

I'll throw out something different here ........ what angle is your seat at relative to the ground??

If You are on a trainer when this happens, is the front of the bike raised up to the level of the rear wheel in the trainer? (i.e. some sort of wheel block for the front wheel).