Is220D Jumping Out Of Third Gear

[Swindonlexus]

I'm a newbie to the site and lexus in general

I have recently bought an is220d on a 06 plate. Occasionally it will jump out of 3rd gear when the engine is cold. Its an intermittent fault and seems to occur when the power comes off when driving towards a junction or roundabout

I took it to the Lexus Independent guy in Swindon and he thinks its a thrust bearing that's gone. Unfortunately he doesn't touch gearboxes. I wondered whether anyone can make a recommendation locally for me to take it to and have it looked at

Thanks