Autopilot gets flakey near end of flight

Ok, so you are 2 hours out of you home field on the way to some new and exotic local. You've read the star and decended to the correct altitude and airspeed (12,000ft 250kias) by the required waypoint and checked in with the approach controller on VATSIM. Approach starts feeding you vectors any you dutifully read-back and set the AP. Course change... fine New altitde... set (hear engines throttle back). Another look at the checklist... set 210 KIAS. Speed bug looks good, another course change and then the call... XYZ123, ABC_Approach, say speed? You look down and you're tooling along at 275 kias getting ready to turn final! 😳 A quick scan of the AP: FD on, AP on, AT on, 210 kais set and on, 4000 ALT set and on, HDG 230 set and on. About then the controller pops in private... Spoilers Buddy! So you turn off the AT, AP then slam the throttles shut and deploy the spoilers and then... Nuth'in. You are still tooling along at 275 way past the turn onto your approach...

I've had other approaches like this, takeoff OK, En-route no Problems, Get near final and the airplane starts getting flakey.

The aircraft was a Mike Stone GMax 757-200 with the default 777 Panel.
Could this be a frame-rate issue? I did not percieve any visual hints of degraded performance.

Any ideas would be welcome.

Chris