Your mention of a ramped gate voltage tends to confirm this. Investigate the gate drive requirements for your transistor in detail. You can use 6N137 together with driver.'Gate charge' datasheet plots can be very helpful for this.Additional comment: to summarize, you may need a much higher current capability gate drive circuit (both sourcing and sinking) in order to obtain reasonably fast switching and gate drive waveforms. It provides 9A peak current and is capable to operate in quite high frequency range. I assume the 6N137 has an open collector output.There is a modification with 2-fet output cascade.
What's the value of the pull-up resistor? If the gate resistor is too large - or maybe missing! - you will have a large time constant together with the gate capacitance and that could result in a ramp voltage at turn on.
