If the slow release version still doesn't work for you, you could try asking your Dr about Glucobay.
It is more expensive that Metformin but has a different way of working (it slows down the breakup of carbohydrates in the gut and doesn't get into the bloodstream) - some people do get some stomach gas symptoms, but it is often much better tolerated than Metformin.
I was on it for a while (because Met gave me problems) and it was fine.
Important to note that Glucobay *only* works by slowing down conversion of carbs to sugars (thus limiting spikes), while Met has a more direct effect on both insulin resistance *and* regulating the way that the liver releases excess glucose.
Depending on the stage of progression of diabetes in your specific case, it may not be suitable anyway. That said, it can be used on its own or in conjunction with Met - if you are on high dose Met, it might be that a very low dose Met (250mg per meal) and Glucobay might control your sugars without quite so many trips to the loo...
Only your Dr can say, of course.
best of luck anyway.
mark.