Yes, but actually there were about 185 million years between the end of the Pennsylvanian and Permian, 80 million years between that and the Ordovician and Silurian, and 170 million years between the Ordovician and Silurian and late Proterozoic. As you mentioned, tens of millions of years of an ice age are spent building up momentum for positive glacial feedback.

I admit, "the earth had existed for several hundred million years without an ice age" is stretching it just a bit. I should have said a couple hundred million years rather than several hundred million years.

Dave