1980 450 SLC Engine Stalling after long run

Hi Folks ,
I recently bought a 450SLC in reasonable condition with a view to restoring.Motor starts fine & goes well.
I took it for a long run a couple of weeks a go & covered about 150kms without incident until I came back to town & stopped at traffic lights. The car started jerking when I took off & started loosing power. I managed to get home with problem slowly getting worse.
I turned the engine off & restarted it a few minutes later & it seemed to be revving cleanly again , but didn't take it out again until a couple of weeks later.
Car drove fine on highway until I turned off to back street & problem arose again. This time power loss was greater & I ended up calling roadside assist . Noticed idle was high prior to switching off. When help arrived symptoms seemed to have gone away. I was told brake vacuum booster was leaking & that this may be affecting the air mixture control for the fuel injection . On reflection the engine problem appeared after the brakes were applied. Are the two connected ?
Any assistance in tracking down the problem would be greatly appreciated.

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