series 1 engine idling rough and struggling at low revs.. Ideas?

[smokey]

Alright guys.. Here's one for the tech heads out there..
I've got a 91 VG ute that has recently had an engine replacement (series 1 w/132000 kms)
Myself, a mate and a carton of tinny's did the job ourselves about a month or so ago, and I've been tinkering around with it a bit ever since because I just can't seem to get it to run properly, the engine itself is immaculate with a full service history since new (I have the log book from the berlina that it was removed from).. However it has always idled a little rough, I know the series 1 engines had a bit of a rough idle but this is worse than it should be..
Coughing and sputtering a bit on first start up, then then getting a little better as it warms up, then at running temp it's like it surges every few seconds it almost stalls then comes back up to idle again, which tells me ignition related problems??
So what I have done is replace coil packs, module, plugs and leads.. still no change...
When you knock her into gear and take it for a drive, a similar thing happens, at low revs (just idling off the line nice and steady) it coughs and splutters again like its struggling to move, then if you give her a little stick it runs fine, so I thought maybe fuel related??
Before putting the new engine in I had put a new fuel pump, filter and replaced all rubber lines.. Then just today, I swapped injectors and regulator off the old engine (which was running perfectly before I shot the big end)
I've also swapped computers and even tried a couple of different chips but I still can't get it to run properly.. So I'm just running into dead ends now..
I have 2 complete engines here and a parts car so I have plenty of parts, even just to try them out, but I'm running out of time as I have to drive over the coast on the weekend, and I'd really like it to be running properly before I take the 4+ hour trip, so any help at all would be greatly appreciated...
I'm sure it's something so simple that I'll kick myself for missing, but sometimes it helps to have a fresh lot of ideas on the job..
Thanks in advance guys, I really appreciate anything you can suggest

[mr calais 5.0]

Have you cleaned the throttle body? Could be the tpc as well. Mine would idle ruff down low and splutter and die. Had to keep ya foot on the gas to keep it running..

[smokey]

I havnt cleaned the TB yet, It's next on the menu though.. I will update if theres any change.. Thanks

[hi_ryder]

whats the build date on the engine? the early vn engines (pre oct 89) run a a different memcal tune. this might not be why the engine is running rough but you wanna make sure that you have the correct memcal (or whole computer) as a starting point so you can chase down the real problem. pre oct 89 have diffrent cam, timing, double valve springs and some other stuff thats memcal specific...

[smokey]

Yeah I have the computer that came with the engine fitted, and I did try another from an 89 VN (not sure of build date) but it made no noticable difference..

[smokey]

Thanks for all the help fellas.. turned out to be a dodgy oxygen sensor.. runs like a dream now, I spoke to a mate that works at the local holden service department and he said it's a very common problem thats often overlooked by a lot of 'backyard' mechanics.... Would have loved his input about a month ago, but I'm glad it's running well now.. I hope this thread helps anyone else with a similar problem