Nicehash is penalizing rapid pool jumps AFAIK. We are talking about hash-power peaks in periods of tens of seconds sometimes (the graph in the picture is an average in longer periods). But yeah, I suppose it could very well be so. But more likely some dude with some hash-power more than "the rest of us" small-time miners.

I wasn't talking about ASIC miners or such, but more about system vulnerability.

For some obvious periods of burst-time there has been hash-power collecting parts of blocks, sometimes several blocks (and block rewards) per minute, and this just at one pool (the second in order of "the big three" AFAIK). This should be easily enough to check in the block chain.

One block every 2.5 minute, right?