[Nexus6]

Hi everyone from the UK. I own a 2000 Boxster S with 111,000 miles on it. Have been meaning to join for a while but recent events have made me get on with it. I hope you guys have got some ideas...

Last week I had all my bushed parts replaced. Control arms, drop links, (ARB bushes not replaced) tuning forks, inner and outer tie rods and so forth. I then I had a full alignment done. Now though its started squirming like a bastard at the rear. This is when it goes over undulations and bumps , changing lane on the motorway. At first I thought its new bushing and its 18 inch tyres tram-lining so its normal but in the wet this morning its frankly scary. At one point going pretty straight on the motorway ( and I wasn't going fast) I thought the back end was going to go for a second. The tyres are PS2's and under a year old. Tyre pressures checked and appear ok. What could be causing this as I didn't have this problem before and I've seen the bushings on the old parts and they were in a poor state.

New parts interacting with old shocks? Old ARB bushes? I was told the shocks weren't great but not terrible.
I was told rear tuning forks didn't need doing? I'm now suspicious of this.
Old top mounts?
Bad alignment done by Porsche. Seems unlikely but needs mentioning.
Some thing loose on the new parts?
Just wondered if this was symptomatic of something well known on these that I'm just ignorant of.
Sorry to start on a downer everyone but needs must.
Thanks to you all