Thanks for the responses so far,
Pulses are from a data stream provided by external hardware. I'm working on a project with WS2812 pixels, data is sent serially and each pixel snips off its own bit of data then passes on the rest. I want to do the same and remove 'n' pixels from the data stream and then pass on the rest.
In addition I want to be able to have the Arduino do its own test program, but that's the easy bit. External switching wouldn't do what I want, as hopefully is now evident.
Its something I could do in assembler on a pic chip, but I'm much less experiences with C on the Arduino platform.