Porpoising on turns and strange KIAS readings

[N562Z 4]

I'm getting a strange and undesirable porpoising effect in turns with Mike Stone's Gmax 757. No matter which airfile I swap it out with the bobbing persists and I get strange, unusually low indicated airspeeds of around 105-115 knots on final. This can't be right. I fly heavies all of the time and expect nothing less than 145-155 final approach speeds normally, with no problems in any other aircraft in my hangar. All software and drivers up-to-date, including FSUIPC, and I usually have FS meteo running during flight. Anyone else experiencing this? Anyone have any ideas? Thanks in advance!Alex CN562ZMinneapolis, MN