How long have they been in the bottles...that is the MOST important question to ask when someone says they have inconsistant cabontation or gushers more importantly than those silly pet theories about sugar solutions, of similar densities not mixing during basic racking.
If it's been under three weeks, then it's gushing because the co2 isn't in solution fully and is coming out before the beer is really carbed, hence the fact that you say it's flat after.
We get this all the time from impatient folks who open their bottles WAAAAAYYYYYYY early. If you opened them at three weeks, or more, you never would have noticed.
If you watch Poindexter's video on time lapsed carbonation, you will see that in many instances, before a beer is carbed it may gush, that's not from infection, or mixing of sugars, but because the co2 hasn't evened out- it hasn't been pulled fully into the beer. Think of it as there's a lot of co2 being generated and most of it is in the headspace, not in the beer, so there's still "over pressure" in the bottle, so it gushes when it is opened.
But when the beer is truly carbed it all evens out, across the bottles.
On the other hand. If you've waited at least 3 weeks, and all the beer were fully carbed and conditioned for awhile, THEN they started gushing, you more than likely have a late onset infection. Either your batch is infected, or you have some issues with bottle sanitization or your bottling process, and you need to deal with the infection. Check your spigot and bottling setup for issues. Or in your fermentation process.
