The NYSI, the McClellan Summation Index, which I follow almost religiously, has been declining since June and diverging with the main market indexes since August 16. For those not familiar with it, it is based on the NYSE advance/decline line, measuring general market breadth. Its decline is likely showing the gradual stock price decay, of which you write, since June, and particularly since August.

By the way, this current pattern is almost identical to the NYSI in the summer of 1987. When the market crashed in October it was no surprise for anyone who followed the NYSI that year.