5 ohms for those resistors is way too low, and 360uF (makerdino didn't mean mF -- he may have been mislead by the obsolete abbreviation mfd) is not a common value capacitor. And the durty cycle is not what you want.
try 8k2, 2k5, and 0.47uF
The frequency is 225Hz, and the duty cycle close to 80%
For a frequency closer to what you want, two 0.82uF capacitors in series (giving 0.41uF) will give you a frequency of around 260Hz.
The actual frequency will be most influenced by the capacitor which will (if you're lucky) have a tolerance of 10%