Hello,
'03 GS300.
I started to niotice the first problem last year, and it is: when I drive 50km/h in cities (speed limit) with cruise on (not touching the accelerator pedal at all), perfectly flat terrain, straight road, it dcruises around 1000-1200 RPM or so, I think in gear 5 maybe, then it seems to suddenly shift back to maybe 4th, and the RPM goes up to like 1600 or so, then it shifts up again and rev goes down, then up, then down, like every second. As if the car couldn't decide which gear is right. doesn't always happen, but seems to be more frequent recently. What could it be? Oil was changed by the dealer (so only partial change, no flush) with the filter about 50k km ago. Oil is bad? Clutches worn? Some sensor or solenoid acting up? Some "learnt" thing? (These gearboxes do learn drive habits, or is it just a myth?) Went to the dealer, they said, try to "reset" it first, before we do anything costly, but then they said they can't reset it, sorry... (Facepalm)
The second thing, is, when I drive 90km/h or something, and start to slow down by braking, to enter a roundabout, the RPM goes down to like 400 (I wonder how is it not stalling?), and looks like the gearbox doesn't shift back? And when I start to give the AP a little push, to force it to shift back or rev up a little, it doesn't do anything. Only when I push it a little bit harder, then it does shift back and obviously tries to accelerate, which is quite dangerous in wet roundabouts.
What do you people think, what could the issue be?

 

Dirty or Faulty MAF sensor could cause this.

 

I have replaced it last year. Could be dirty again?

 

what is a MAF sensor?

thanks

 

MAF = mass air flow

https://i.ebayimg.com/images/g/Hg8AA...mfH/s-l500.jpg

 

Could also be your fuel pump. Before mine went out I was in cruise control and the car seemed as if it was searching for a gear. Having to press the pedal further for the same speed also points to low fuel pressure. You can have a fuel pressure test done and that will tell you.