Not sure whats gone wrong. I've been using the 707 Captain for the last few years with no issue until AS16 uninstalled FSX forcing my first reinstall.

First flight in the 707 on a test flight from Boeing Field to Tokyo and gauges have frozen up when I reached cruise altitude.

They're stuck at FL291 290kts/mach 0.741.

Any idea whats gone wrong?

Sometimes all the gauges become unresponsive in FSX-SE, but it's a matter of just resetting the flight or restarting FSX.