I have tried it with Pitot heat on and flew twice for around 2.5 hours each flight and never got the oscilation, so thought this may have been the problem until today returning to base to rearm I noticed the air speed and alt numbers on the hud were "stuck" even when decending and slowing down with speed brake. Again once I started my decent from around 25,000ft to land the osilation happened again. Air speed and alt were still "stuck" until I was downwind to land (which then the airspeed indicator on hud went to 500kts then dropped back down to the actual 300kts I was doing) Rad Alt was working correcty , so a possibility is the pitot heat isnt working or needs to be tweaked?