Colorado diesel goes into limp home mode

Runs ok if the revs are kept up, but will drop into limp home mode at idle
while running seems to have full power and normal performance
in limp home mode timing changes (audible) and max revs 1500
compression test, all ~500 psi
has had petrol
error code p0008
engine management light on all the time
service light on but does not come on immediately after reset