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My Dads Vauxhall Vectra is a manual with cruise, although it too is a diesel. I stress tested it on a hill and found that if the going does get too tough, it just drops out of cruise and effectively stalls!

I have the same car and like your dad's, some hills are too much for the CC and it disengages. I actually prefer that the CC doesn't allow itself the equivalent of full accelerator pedal input - I want to be the one who makes that decision. I don't recall it ever disengaging on motorways at normal speeds which is where I use it most. On local journeys in 30mph limits I keep it in 4th with CC on and sometimes 'assist' it with my right foot, if I didn't the speed would drop below the 20mph threshold where CC disengages.

As far as the car stalling goes, you'd have had to have fallen asleep at the wheel for that to actually happen!