With all the beer I have made, there seems to be a 2-3 week period where it is perfect. After that, it is way over carbed.
Then you have an infection issue more than likely. If you've checked to make sure fermentation is complete, left suffficient time (REGARDLESS OF WHETHER IT'S IN THE SECONDARY OR LONG PRIMARY) for the yeast to do it's thing. Added ONLY the correct amount of sugar (weighing is best, because there's too much variability in measuring dry ingredients by volume, try this experiment, take a measuring cup, fill it with brown sugar, then get another cup fill it with brown sugar, the PUSH DOWN THE BROWN SUGAR and fill it with more brown sugar and tell me if it's the same amount of sugar in there. Or fill a measuring cup 3/4 of the way with white sugar, then whack it on the counter a few times till it looks like it's gone down and then add more sugar to get it back to 3/4 of a cup, and tell me if there's the same amount of not of sugar in there.) And waited a minimum of 3 weeks with the beer at or above 70 degrees, if it THEN has been fine for a few weeks but then starts to gush, then you have something consuming unfermentable sugars in there, which means an infection.
Carbonation, like fermentation has a finite stopping point, determined by the tiredness, and or type of yeast (attenuation) AND the amount of
fermentable sugars present to be eaten by the yeast. In the case of carbonation the amount of co2 is determined by the amount of fermentable sugar added at bottling to a beer in which the fermentation is already complete. The yeast can ONLY in normal situations, eat the sugar given it, and nothing more...and therefore only produce a finite amount of gas, which takes about three weeks, and then when a beer is fully carbed, THEN it should NOT carb any further, because there should be nothing else fermentable in there, and therefore no more gas should be able to be produced...
If you are stable for awhile but then suddenly more co2 is being produced in the bottle, and there's NO fermentable sugars in there, then the ONLY thing that could be happening is there is something either eating unfermenable sugars, OR converting the unfermentable sugars into fermentable ones to be eaten by the yeast, and that is an ABNORMAL situation, which means there's a micro-organism in there other than standard yeast messing with things...which means an infection.
Whether you secondary or not has nothing to do with this, whether you cold crash or not has nothing to do with this.
The only things of relevance is 1) Are you sure fermentation is complete before bottling (which usually if you leave the beer for a month, more than likely it is done, especially if it's in primary where the potentiality of not racking too soon and possibly sticking the fermentation can't occur OR making sure after racking that fermentation didn't restart in secondary.) 2)Using the right amount of sugar (which is hard to do without measureing. 3) Did you leave the beer sufficient time to actually carb (minimum 3 weeks) and 4)Was the beer actually carbed for a period of time AFTER it reached the right carb level, BEFORE overcarbing?
Bottle conditioning is really foolproof under most circumstances, especially if you follow what I mentioned above. You add the right amount of sugar to finished beer, and wait the right amount of time, and everythign should be fine. If you've met those conditions correctly, then the only thing that could cause a late onset gusher is an infection. BUT if you've strayed in anyway from what I wrote, then you could have problems, or if it's too early, THINK you have problems.
Everything else is really irrevelent to the discussion at hand.
