How are you making sure that the top and bottom (high-side and low-side) transistors are not turning on at the same time? Because of the internal capacitances in the devices they will not turn off immdiately, so unless you put in a delay between turning one off and the next on there will be some overlap.
The other big problem can be getting enough drive current into the gates/bases to ensure that they switch cleanly.
Usually the best solution I know of is to use a high-side/low-side driver driver IC that is designed for the type of devices you are using. Many of these will allow you to set the 'dead time', the delay betwwen switching the devices, typically by changing a resistor value. Have a look at Farnell or RS for the range of devices they offer.
Good luck! And stand a long way away from that 700V!
Andy Millar CEng MIET CMgr MCMIhttp://www.linkedin.com/in/millarandy
"The aim of argument, or of discussion, should not be victory, but progress." Joseph Joubert