I think I found a bug in the Power Spark module. I've searched the forums and didn't find anything related to this, so here goes, and forgive me if it's been asked before.
I was using an old version of Pitbull (don't remember exactly which). When Patch 2.4.2 came and I had troubles, I updated to r73914.3.zip . All was fine, except that the 5 second rule spark wasn't working properly.
For example, when spamming instant casts: the first one would trigger the spark; the second one wouldn't, and the spark would continue moving as if nothing happened; then the third would (correctly) reset it, and so on. It seemed to happen randomly (e.g. sometimes a string of 3 casts would work, then several wouldn't).
I found that overwriting the /Spark/Spark.lua file with the one from the old release fixed things for me (so far, at least... ).