Ethereum suffers from fees volatility, with a peek of gas spending of $2.7 millions just for the day of july, the 2nd. That day, In order to wait less than 30 minutes, users had to pay more than $3. This network congestion was apparently mainly caused by a single actor, FCoin, proposing services for users to optimize their to optimize their fees
In order to fix this, the Ethereum founder Vitalk Butterin recently (July 2018) proposed a new pricing mechanism for gas fees. Which implementation has not been acted however.