Hi Alexander, Wow! It just goes to show, you get a better class of collector with an AC 428 Frua!
Big Al at Thunder road used to work for us when we had Unicilp Automotive, and we restored several Fruas. He is well experienced in suspension set up as Nik suggests. Whilst it is worth checking there is no play in the suspension joints, there may be something else to blame. What you describe sounds more like a diff problem. I had the same problem with our own Frua. On a 3 lane motorway sitting in the centre lane, kick-down would result in a swerve to the right taking us into the next lane! Lifting off brought us half way back! We matched up tyre diameters and checked all the suspension without any improvement. Then we lubricated the plates in the Salisbury Differential, and bingo it was cured. After that it ran straight as an arrow whatever we did with the throttle. I hope that helps. Andy. PS if that works, I'll pop over for a beer!