Just had my 300Tdi gearbox recon'd and vehicle now appears to 'struggle'/vibrate high up in fourth and low fifth with heavy throttle (can see the steering wheel vibrating). Speed is between 45-55mph. Problem diminishes during 4-5th up-change. Seemed to ease a bit after 20miles. Gearbox oil is ATF. Had new BFG's and wheels balanced 4months ago so don't suspect this. Any ideas what's going on?? Is it the gearbox or something else? Much appreciated.