Within 60 days of a filing under ORS § 759.052, 759.054 or 759.056, the Public Utility Commission shall either determine the appropriateness of the filing or determine that further investigation is necessary. If the commission determines that further investigation is necessary, the commission may suspend operation of the filing for a period not longer than five months from the end of the initial 60-day period. Upon a showing of good cause, any party may request extension of the suspension period for an additional three months. [2005 c.232 § 11]