Yeah, that's what very fast diff retarget seem to cause.
First day: Why should I pay 1000 if I can just wait until the diff drops and some freshly joined miners will be happy to sell for 800 tomorrow? Second day: the same thing with 800 and 700. And so on...
The cycle might never stop because there is always lower and lower ... until ... the coin hits a 0.0x Gh/s speed and thus practically dies.