It is so overloaded orders got $5 to $10 slippage 
why are you using market orders?
When price is rising from 90 to 120 in 10 minutes the market order is the only way to buy. Unless you know a better way that doesn't involve black magic...
You can always limit order above market price.
That's market order
At the very least it caps your slippage.
That in english translates to "at the very least prevent you from buying".
Try this little experiment: put a market order when price is 90, and a limit order at 95. A few minutes later price is 100 and the market order gets executed, but the limit order will be still there. market order -> slippage, limit order -> no buy
Now I don't know if Gox works in a weird way, but this is how it is on bitstamp, market orders get always executed first, and not even all of them.