Your sim isn't frozen, it's just running very slow since you have so many bots. If you think about it for a moment, what is happening is simple to understand.
The bots were laboring under heavy cost pressure, meaning they had some strategy that allowed them to surrive in the face of siginficant costs. Since bots can eat faster than the costs can whither them away, finding food becomes the main determinant of survival. A common strategy for finding food is to reproduce a lot. You get a high moratilty rate, but you get a lot of offspring looking for food. A few will find it. But then something happens - all the veggies are on the far side of a shape or somethign and the population drops sufficiently to hit the zero cost threshold before the CostX multiplier can drop enough. CostX hits 0 and suddenly yoru sim is full of bots who reproduce like crazy and there are no costs! THe popualtion goes through the roof and CostX doesn;t start getting ratched up until the population exceeds the upper range. But it's trajectory is ballistic and it way overshoots. It will come back down once CostX gets high enough, but it takes a long while since the sim runs so slow with lots of bots...
The forthcoming costX reinstantement feature should address this.