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Yes, start by checking the cruise control port at the diagnostic connector (port 2 of the B connector) as described in the FAQ.
For cutting out at high speeds, and possibly also the excessive initial surge at engagement, consider a vacuum leak. As vacuum drops when accelerating and at higher rpm it could be causing the cruise module to deactivate just as if you were pressing on the brake pedal and tripping the switch. If it's leaking, it may not take much drop of vacuum to cause a disengagement. I take it you've got a turbo with a vacuum pump. If that's the case then that changes that scenario a bit as vacuum is more constant, so the leak would have to be more significant in that scenario. Engine movement during acceleration may also be aggravating a split hose that is more sealed under static position. Apply mouth section and seal it with your wet tongue for a few moments to see if it can hold vacuum. The leak could also be inside the cruise switch at the pedal. A length of vinyl tubing could be used to test the switch by itself to see if it can hold vacuum.
The pedal switch is also adjustable and may be set too close to the hairy edge such that any slight movement of the brake pedal causes it to disengage or refuse to be engaged. I've had that happen when I was playing with the switch to trip the cruise control out without illuminating my brake lights to following vehicles without having to take a hand off the steering wheel and use the stalk switch. In your case I don't see how simply getting above 52mph would cause such pedal movement.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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