Shock absorbers shagged, radius arm bushes likewise, swivel pre-load too low, steering ball joints shagged or anti-roll bar bushes going soft.
Mine does it too but at higher speed and I'm working my way through the above list. Shockers massive improvement, roll bar bushes cheap and easy, found some play in my drop arm ball joint so I'm doing that next week after that will come swivel pre-load check / re-shim. It's always the last thing you do, so mine will be Radius arm bushes, which seem a little soft.
Hope that helps.