Thanks everyone for the comments. I will be purchasing a fuel pressure gauge to monitor the values. It's a relatively inexpensive and useful item. Will keep everyone updated on the results.
 
On another note:
 
Doug kindly provided me with his MAP sensor to test in my car. Stumbled 2 out of 3 first starts now. The new circuit opening relay arrived this afternoon so I will drop that in the next few days to test.

Thanks for the updates Lumix.  Easy fix if it's just the COR...

As we discussed give it a week and then swap the old COR. back in and see if the stumbles return again.
How did you go with fitting the fuel pressure gauge ?

Update
 
Car stumbled after 16 days straight of perfect starts... The only thing that had changed was the newly installed fuel pressure gauge and fuel filter. After installing the new fuel filter and gauge, fired up perfect, drove the car around later that night, parked it up and started it 6hrs later on Sunday Morning to meet the MR2 QLD club and it stumbled. Fuel pressure before starting was 0 psi as it was stumbling the pressure was steady at 44 psi.
 
This morning (Monday) fuel pressure was 0 psi, the car was started while I watched the gauge. I had instant fuel pressure to 44 psi and the car started perfect.
 
Reading through the service manual at the moment to check the specs

Mrskylighter
Leaky injector maybe as fuel pressure drops to zero overnight? Too much fuel in a cylinder causing a temporary misfire at cold start?

Maybe see how long it takes for the pressure to drop?

Bummer, I thought you fixed it.

I had the injectors tested, cleaned and serviced in April last year, no leaks. I will monitor the pressure drop.
 
Same, I was starting to get excited.

Decided to check the fuel pump operation.
 
I can confirm fuel pressure holds well within spec after the motor has been shut off (38 psi). Manual specs after 5min fuel pressure should be 21 psi or more. 
 
I followed the service manual for checking the fuel pump operation
 
- Connected the terminals +B and FP. Turned the ignition switch ON.
 
Fuel pressure showing 46 psi, (manual lists between 38-44 psi)
 
- Removed the SST (paper clip) and started the engine and disconnected the vacuum sensing hose from the fuel pressure regulator and plug the hose end.
 
Fuel pressure showing 46 psi (Manual lists between 38-44 psi)
 
- I reconnect the vacuum sensing hose to the fuel pressure regulator
 
Fuel pressure showing 46 psi (Manual lists between 30-37 psi)
 
- Ok now I'm thinking my regulator is not operating correctly. Doug is kind enough to take my phone calls and answer a few questions. I connect the vacuum line from the fuel regulator to an area that I know has vacuum. Start the engine.
 
Fuel pressure showing 38 psi (Manual lists between 30-37 psi)
 
- Confirmed the fuel pressure regulator is operational. So that means my vacuum lines may be connected incorrectly. I begin to check the manual again to find more information about the EFI system. The manual in the EFI schematic/description shows a VSV for fuel pressure and VSV for ACIS.
 
My car only has 1 VSV and it's between the vacuum tank and Actuator (VSV for ACIS?)...
Fuel pressure regulator vacuum line is connected to the main air intake hose T-piece. What has gone on here? I can't see any uplugged electrical connectors loose anywhere for a second VSV for fuel pressure.
 
Scratching my head at the moment.

Mrskylighter
Are you using the below 3sge manual? Looking at EG-234 it certainly describes a VSV being between the fuel reg and the vacuum source.
http://www.sq-engineering...kshop-manual-early.pdf

Yep, that's the 3SGE manual I'm using. Page EG-163 shows the EFI system schematic too if your interested.
 
Labelled the vacuum hoses and snapped a quick photo under the car.
 

 

 

Currently my fuel pressure reg isn't connected to a VSV. Fuel pressure reg vacuum line connects into the intake hose. But if I connect the fuel pressure reg to a different vacuum source the fuel pressure dropped to 38psi. The second photo shows the VSV (Green colour part) which is connected to the vacuum tank and actuator. The manual shows that there should be a second VSV mounted next to that one which the fuel pressure reg vacuum line should be connected into.
 
If someone with another later model Bathurst could check how their car is set up and post some photos for reference. I'm not sure if Toyota decided to make changes mid way though the production line and removed a VSV...