What 555 timer circuit are you using? Is it like the one below?
Here's the one with the comparator, which tooki was talking about. I don't know why people use the LM358. The LM393 should be used instead, because it's a proper comparator. The LM358 is designed as an op-amp and performs very poorly when used as a comparator. A MOSFET driver may be required, if the turn on time is too slow.