Got similar problem with '97 Verada 3.5L.
I'm stumped!!
Still coughs & splutters on acceleration..
Replaced:
Spark plugs & leads (new)
Dizzy cap, coil, transistor module & rotor.
Air flow meter
Throttle position sensor (new)
Fuel pump
Compression test, all good and above spec. of 1200Kpa
Manifold vacuum 60Kpa on spec.
Cleaned & checked injectors.
Found corrosion on ECU pins from heater leak, cleaned pins and sprayed CRC 2.26 on plug & sockets
Idles OK but still coughs & splutters on acceleration.

I had this car once that idled great and was 100% perfect up to half throttle. Tried everything you e tried and then some.

Turns out the cat converter had collapsed. Check yours for rattles maybe?

Another thing is to check your temperature sensor is within spec' - check the resistance over the two pins at different engine temps and compare the readings in the workshop manual.

You replaced the fuel filter with the pump I gather?
That is the the usual insurance policy.
With all the things you've done so far,
the heater leak might be the clue.
I think the ECU is under the seat?
If so have you taken the lid off to check there is no coolant in it.
Your prob sounds like advanced timing to me.
See how you go and post result.

Another thought.
Go back and make sure you haven't got the firing order wrong with the HT leads.
Spluttering idle and revs sounds like wrong FO or a timing prob, too advanced.

Thanks for the ideas guys & moderator putting my post in appropriate forum.
Heater been bypassed since I broke the plastic pipe off at firewall changing heater hoses.
Cold resistance of temperature sensor 2k2, on spec.
ECU lives on middle floor under radio, heater leak must get there by ducting.
Motor revs smoothly above 2,000 RPM. wouldn't of thought in would idle & rev OK if firing order was out but I will check again.
Have had lid off ECU, no corrosion inside only on a couple external socket pins, flushed under hot water, dried & sprayed with CRC.
Question, If I replace ECU will it work without replacing anything else key/barrel?

If the engine might be running rich, the one thing that I don't see listed as being checked/replaced is the oxygen sensor. This can be checked easily by disconnecting the sensor and disconnecting the battery to reset the ECU. With the sensor still disconnected, take the car for a drive and see if the behaviour improves. If it does, further investigation of the sensor would be warranted. If not, reconnect the sensor (with the car switched OFF!).

Haven't tried Oxygen sensor yet. Going to plug ECU into mates car.
Won't be able to try for a couple of days

Update: Tried my ECU in mates 3.5L with Tiptronic - wouldn't start at all.
Tried 6G72 3.0L 4 speed ECU on my 3.5L 4speed - wouldn't start at all.
Have Oxygen sensor to try.
Temporary fix - went out and bought cheap 2004 Verada for $3K3.

strangely after car got serviced in dies in idle instead of dropping, whats the go.

Oxygen sensor checked OK, timing OK, only leaves ECU or wiring problem.
This is driving me crazy about to flog it for what I can get.!

Another thing I just noticed after re-reading the thread: you don't mention the throttle body or idle speed controller (ISC). It might be worth checking the TB out and if necessary giving a good clean. The ISC stepper motor may also be playing up and could be worth checking too.

Edit: swapping ECUs also requires dealing with immobiliser issues - 97 and earlier 3rd gens don't have reflash support. Reflashable ECUs can be used in them but neutralising the immobiliser requires reflashing which would have to be done out of the car. H and later series ECUs will also throw CELs due to configuration differences, though the car will run okay.