2000 Vauxhall Corsa LOSS OF POWER AT LOW REVS WHILST BRAKIN

Engine Performance problem
2000 Vauxhall Corsa Front Wheel Drive Manual 50000 miles

When I am braking to a stop at low speed and low revs the car looses power and stalls once I have come to a complete stop. It also looses power when I am pulling away from lights in first gear but only briefly before kicking in again.

What could this be?

Thanks