Hello,First time here looking for help. 

Subaru impreza 2lt non turbo sport wagon. Drives fine generally until travelling 10miles at a steady 70 to 80 mph then develops dull knocking sound which turns into a violent shuddering of the whole car, After a brief stop everything is all ok?engine in good order,tyres all new and balanced goes into and out of gear fine.  

 Can anyone shed light on this please.

I had a very similar problem recently and it turned out to be a buckled wheel. U say you've had the tyres balanced but did they check for buckles. I had it for ages and while i had my car on axle stands one day my son was spinning the passenger wheel and i noticed the wobble on the drivers wheel. Only slightly but it was there! Changed the wheel and boom!! Problem solved. Does your car seem to have a limp at very slow speed??

Very possible what rallyally has said and is excellent advice. I think it could also be warped brake discs or bearing. If the bearing has gone the wheel once jacked up will wobble or feel loose when you rock the wheel up and down.

many thanks for advice, Car drives true and straight with no detection of buckled  wheel same applies to the discs with no patter or juddder through the pedal. Have had new bearing in osr and checked the nsr which seems fine. 

 Its def a weird one as driving the car out and about all is good until a long maintained speed of between 70 and 80 then all hell breaks loose!!

     Cheers for the comments thou. 

Have you checked all the other parts diffs serviced StrutsTop Ball jointsTierod ends Bushings.

Cheers for that,1st job tmoro. I'm thinking either uj on rear drive shaft /s or diff.

Soinds like a fun day for you then :) but least the weather is meant to be really hot so least you will have the weather for it [emoji29]