you say when the motor is not there the voltage is correct? since it will be pulsing how are you measuring it?
If it is a DVM then ok the average voltage would come out.
The addition of a high value capacitor (with sufficent current ripple rating) AT the motor +ve terminal will provide tank capacitance to the motor and make the voltage supply look more like a voltage source than a current source.
Also add a low ohmic 10Ohm in series with the Gate, this will stop the creation of a pearce oscillator..
Also that diode, is its characteristics matched to the FET's switching performance, if it isn't you could get turn-on problems
Since it looks like you are wanting to maintain the voltage I would suggest you put a close-loop around the system to maintain the voltage. This way you will be able to maintain the voltage. Remeber a motor is in effect just an inductor.
As to a snubber for this type of circuit with this type of voltage/power it will not be needed.
A snubber is only used when switching losses are such that it can damage the switching device or the turn-off voltage overshoot is very high that it will kill the device.
Snubbers are the very last resort. Better layout of power devices with low-inductive busbars are always better. All snubbers do is move switching losses from the device into the snubbers R component..