It's been my impression that most miners are selling it for cost + small margin on top.
It's not the case. Less than 1/3 of the miners do like this. See
https://bitcointalk.org/?topic=86010 (people can put 2 check marks there so numbers don't add to 100). I pay my mining expenses from my pocket and don't spend my mined BTC.
The biggest source of volatility in the price of bitcoin, seems to be the speculation done by investors which pump the prices up.
Speculators don't bring more volatility but reduce it. If you buy low and sell high you shift the price closer to equilibrium, not away from it. And many speculators cannot pump price up - some will try to pump up, but other people will try to capitalize on that attempt. I think the only way of "artificially pumping" the price is one big manipulator putting his $200k+ at risk. A quite brave man indeed.
I believe the prices have been stable for the last couple of months because of previous crash that might have scared away these speculators.
I think quite the contrary: I think the price was stable because the only people trading in those dark days were speculators, "the sharks" as someone put it. And teenagers and housewives were removed from the market and attracted to ZhouTong/SatoshiDice type of gambling which is more or less closed-circuit and doesn't affect the market.
]Honestly, I think it is better for the bitcoin future if the prices could be remain stable. Because then people would think of it more as a means of buying things rather then buying and holding the currency.
Bitcoin is doomed to a steady exchange rate growth because of a) its algorithmically limited supply b) an irresistible temptation for Federal Reserve and European Central Bank to increase the supply of paper. Gold prices in USD increased 580% in last 10 years, oil increased 320%, corn 400%.
We cannot have stable exchange rate as I described above but we don't need it either. We only need less volatility - 1% daily rate change must be quite acceptable for merchants and service providers. If we target less than 1% of daily growth we can afford to have up to 3800% yearly growth which is a quite high limit. To get less volatility we need to attract more hoarders (which remove USD from their mattresses and put them into bitcoin market) and more speculators (which extract profits from reducing volatility), not less. With more money in the market it's volatility will go down and BTC will be more attractive to real producers of goods and services. The inherent benefit to early adopters boosts Bitcoin, not suppresses it.