ok details 98 Si converted to 2.0 silvertop from mondeo has had ecu conversion too.

i'm using a known good set of leads and have changed the plugs twice.
I've changed the inlet manifold from a 2.0 to a 1.8one
I've cleaned the iscv with carb cleaner
I've fitted a second set of 2.0injectors.
new airfilter (standard paper in airbox)

none of the above made any noticeable difference to the problems i will describe.

(car is decatted as i didn't want to buy a cat - i will in good time no point while it will get petrol soaked
and has a stupid exhaust backbox on it)

when started from cold the car sometimes revs up to 3k and then drops down to its normal idle
the car hunts on idle... not much though it sits around 900rpm but it hunts a few rpm maybe 50odd not much but the stupid exhaust makes it sound like i'm revving it
The car hessitates and there is a noticable kick at about 4500rpm where the car starts to take off, not in the usual zetec way but as though its suffocated up to this point.
the car will not run smooth at low throttle and is very jerky, in every gear.
the car backfires at the limiter... big style
the car pops and bangs during deceleration under engine braking - not little gas pops actual fuel combusting in the exhaust.

it was fun for awhile... all the noise etc but i'm weary of it now, i'll get a cat and standard backbox once it runs right.

any thoughts... i don't fancy taking it to ford due to the conversion etc they'll not fix it and tell me its my fault as its not standard. any suggestions on what to suspect / try next?