Fuel Consumption Gone Up - Subtle Problem

Hey Guys,
Unlike a few other cases, my problem is a subtle one, fuel consumption has increased by around 10 - 20% at a guess. Engine still purrs faultlessly as it always has though the exhaust is quite potent at startup.
My first thought was oxy sensor but speaking with Road and Track they seem to think that wouldn't be the issue as oxy sensor is for cruising only.
Sight unseen they are suggesting the temp sensor playing up causing the auto choke not to release or injectors fouled.
Temp sensor theory may check out as the car is idling when warm around 1000rpm where from memory she usually sits around 600-700rpm. Also the tacho needle is not steady at idle, oscillating between 800-1000rpm.
 
Car is 1996 3SGE with 180 000kms on clock. Can anyone suggest another cause, verify these possible causes and/or tell me how to check them out? Is it worth buying that injector cleaner stuff or is that a waste of money as I have been led to believe many of them are?
 
Thanks in advance.

Re injector cleaner - get one with polyetheramine in it and it will work eg. Nulon Total fuel system cleaner.
 

Thanks for these great tips, going to try pretty much all of them, I don't know how to check codes and/or do most of what Essdub posted. Will get searching but if anyone can point me in the right direction would be greatly appreciated.

purple5ive
 Any evidence to back this up as i alway wanted to try out the nulon ones.

Not first hand but from what others have said, it does work. FWIW I think the Toyota brand inj cleaner has polyetheramine in it too.

TRD2000
afm?

'96 3S-GEs don't have one...

If the fuel consumption suddenly got worse & it started hunting, I'd check the vacuum lines to MAP sensor (often tend to split @ the end - causing a small leak & MAP sensor triggering a rich mixture).