I've just posted in the SC430 forum about the erratic shifting issue I have, you guys seem to have nailed it! I have the facelift late 2005 6 speed that our cars share. I'm also getting some neutral drop out on medium braking. This is my thread in that forum:
https://www.clublexus.com/forums/sc430-2nd-gen-2001-2010/967805-very-erratic-transmission-shifting.html

So could this be the ECU issue? I'm in the UK so sending to SIA Electronics probably will not be an option?? I hate my Lexus doing this

 

I believe the only solution is getting a new ECM or having it repaired someplace else rather than SIA. You could ship it but I'm assuming you can't afford to be without your car for that many days.

 

Sounds like the ECM. I recently ordered springs from a company in the greater London area and with FedEx it took 4 working days; two of which were for customs. The shipping was under $30 USD so it may be worth calling or emailing them to get a quote and if they ship international.They probably have experience in international shipment. Another option is to see if they will source an ECM, test and repair if necessary and ship to you. Hopefully you’re near a major metropolitan area and not in a remote village. I lived in just such a village in Scotland. Aye; no sheep shearing classes.

 

The heat cycle causes your type of problem often when the car is aged cracking solder joints. One idea is to fix it in your garage. It's quick and fast.
https://www.clublexus.com/forums/ls-...l#post11064545