Parsing a flat file, how to create a csv file

I have a very large file that looks (more or less) like:

I: Steve
1234 Oak
Apt 5
City
I: Jack
99 Elm
Town
I: Dave
123 Pine
Suite 444
Mail stop 5
Village

And I want it to look like:

Steve, 1234 Oak, Apt 5, City
Jack, 99 Elm, Town
Dave, 123 Pine, Suite 444, Mail stop 5, Village

Each block of customer data begins with an "I:" and is followed by a name. Basically, if every "block" of data was the same number of lines, say 4, I'd just loop through the file and create a "record" for every four lines of the file. But in my file, some of the "blocks" are 3 lines and some are as much as 10. I know a new "block" begins with an "I:", but I cant figure out . . . well, hopefully you get the idea.

This is a one-time task, so a quick and dirty solution is fine.

Thanks
Steve