engine running rough at low revs

[defa]

my mother inlaws s2 vn v6 is running like crap. it idles carp, runs crap. back fires but as soon as it is above around 2000 it seems to kick in and run ok.
i hooked my laptop up today it and it only came up with one fault and that was 34 which is low voltage from the map sensor. but it does idle at about 700rpm.. i thought that could be the reason..
it seems to have prenty of spark. new plugs and leads. new fuel filter a few weeks again and a new air filter. i pulled the injector plugs off one at a time and all cylinders seem to be firing. just not that well.
any help will be great.

[purrVN]

any trouble starting?

[defa]

nah. starts good. i had a thought maybe the fuel pressure reg was playing up. but i don't have any pressure testing gear here and i didn't have a spare one to try.

[Red Ranger]

Yeah 700rpm is good... Buicks do idle pretty sh!t anyway.. Bad idle+good spark=bad air/fuel..
I am captain obvious,
How dirty is the TB?

[defa]

i should have had a look at the Tb but i didn't. it doesn't just run bad. it runs really crap. while sitting on a 90-110 it misses and backfires when ya sit it at about 1500 it misses, in park that is