Yes, it's probably garbage collection. To be sure, you can run your program with the "-t" RTS option, which will create a file in the current working directory named "foo.stat" if the executable is named "foo". The resulting file will contain the total amount of time spent, the mutator time, and the GC time. (I recently ran into this problem myself...)

The way we normally do this is to add '+RTS -sstderr' to the command line (avoids the pesky foo.stat file). Cheers, Simon