starting problem

when i start my car from cold in the morning it ticks over at about 1200rpm for a few seconds then it just tries to cut out,when you try to rev it,its almost like its stuttering and misfiring
restart it and then its fine again for a few seconds and then dies again
but once its warm its fine
recently changed my o2 sensors for another fault i had and this appeared shortly after they were changed
its a 99 v8
yet another thing to worry about with the rangie

and to top this off when it switches over to lpg it runs very sluggish
it will tick over and rev fine on lpg but pulling away and driving along its losing power