New trials may be granted and judgment set aside for good cause, on motion or on the court's own motion on such terms as the court shall direct. New trials may be granted when the damages are manifestly too small or too large. When it appears to the court that a new trial should be granted on a point or points that affect only a part of the matters in controversy and that such part is clearly separable without unfairness to the parties, the court may grant a new trial as to that part only, provided that a separate trial on unliquidated damages alone shall not be ordered if liability issues are contested. Each motion for new trial shall be in writing and signed by the party or his attorney.