Yes. They come in groups. It's not totally random. There are hidden parameters that are not well understood. In the hidden parameter space there is tide of "luck". I can only guess the parameters are related to network speed, performance of the whole network, and the condition of your peers who are network neighbors at the moment.
Sorry, I can't agree with this, and I have found several thousand XPM in total now. I think you are falling into the trap of seeing patterns where there really are none:
http://en.wikipedia.org/wiki/ApopheniaIf the distribution of blocks is "totally random" (which, btw, is pretty hard to define in itself) then they won't be coming at equal intervals. Think about the following bit pattern, which would correspond to blocks coming in regularly every 4 days:
00010001000100010001000100010001
This is obviously not random, at least in the Kolmogorov complexity sense, that you can write a very short program to produce this string of bits.
If you move the 1s around to make this more "random", then you're bound to see some clusters of 1s, separated by longer stretches of 0s, with the occasional lone 1. However, if the process behind the bits is "totally random", then there is no particular reason why given 1s would cluster up. Yes, they come in groups, but not necessarily because of any hidden parameters.
It's interesting how shorter repetitions are considered lucky patterns, but a more distributed pattern like the one above is not usually noticed, unless you actually keep track over many days.
I've thought about this aspect of randomness in many areas of human life. When things are distributed randomly (though uniformly in a large scale), some people are bound to get more than others. This is why some people are considered "lucky", they are just the ones where things cluster up. If you want to eliminate luck, you need a command economy where everyone is forced to have the same amount of everything.