Vauxhall Astra 1.6 cabriolet loss of power.

hi peeps ive just bought an astra 2002 cabriolet 1.6 and its fine on start up when cold but within a minute or two the yellow light comes on the dash and you get a loss of power even at idle..if you drive it and go to accelerate nothing really happens if you take your foot of a little the car seems to pick up a little and if you are patient you do eventually get up to speed.
and with an obd the codes are coming up as these.
P1120
P1122
P1550
P0136
P1911
any help would be much appreciated thanks steve.