Weird Starting Problem

[Rolls]

Ive got an older R31 Skyline with the rb30e motor in it. Now when cold or left for many days the car will crank once and start instantly with no throttle and idle perfectly and not blow any smoke (not bad for 350+kms) If I leave it for up to 30mins it will start instantly as well however if I leave it for any longer around 30mins - 10hours it struggles to start. With no throttle it will crank for 10-15seconds before it starts or if I stab the throttle repeatedly around 5seconds and will puff a little white smoke and give the smell of fuel for about 30seconds, often the first couple of seconds sounds like its running on only a few cylinders.

Now I've heard dirty injectors could cause this so Im going to try some injector cleaner to see if it has any effect at all indicating a clean would be a good idea. Any other ideas guys? My economy is great at around 10l/100km with all city driving giving it a bit here and there.

Its a cheap daily so I dont spend much money on it aside from regular oil changes to hopefully see this motor lasting as long as possible as it is still in very good condition (want to try and crack 500k).

Help.

Cheers guys.