Steering rack bushes - wear will not show on a visual inspection without the rack being disconnected from the hubs.
Also take a cross measurement - right rear hub to front left hub and vis versa. If these are not the same then it could be a worn bush in the rear suspension.
I know nothing about 150s but had a similar problem years back on my 40 and it was a worn bush. The cross measurement gave me the clue.
I have replaced nearly all the bushes & new steering damper I'm really at a loss I took it to peders for a steering & suspension check they gave me a list that I have already started telling me to replace bits I have already done & said that they could do it for a bill of 7000$ gotto sort it soon I need it for work .