Stomach issues are not usual with plain guaifenesin, so I wonder if you are experiencing a reaction to the binders in the tablet? There are a few generic brands of extended release tablets available, I have found them on-line and occasionally at Walgreens. You might want to try one of those if you prefer a tablet or the 12 hour coverage.

I use 600mg ER twice a day - Since there is no extended release liquids available, you would need to dose 4 times a day. Just be sure whichever one you choose is ONLY guaifenesin - no decongestants or cough suppressants.
Sue