try eating an hour to an hour and a half before your workout. It sounds counter productive i know, but it's worth a shot because maybe when you use alot of force, or concentration in your lifting, your stomach's acid is out of whack due to lack of food. I used to burp and get bad heartburn during my lifts, but I figured out that I needed to eat either a low sodium meal, or have a shake about an hour to an hour and a half before my workout. Good luck.