That may be caused by the garbage collector, using Adobe Scout should help you out trying to understand the problem.
If you don't want to use Adobe Scout, or if you just want to always have a little "benchmark" you can use https://github.com/mrdoob/Hi-ReS-Stats which will a little graph of the memory used and FPS which will help you to discover when the garbage collector "collects" (you'll see the memory used decreasing) and you can detect slow frames without using trace.