hilux with a mind of its own...please help!!!

hey all
i've got a 1990 hilux, power steering, leaf srpung front running 31 10.5 15 pirelli scorpions. if im on a decent stretch of bitumen it will drive pretty straight and civilised but as soon as i get onto an uneven piece of road the car is all over the shop and almost impossible to keep it in a straight line.

ive had it to city suspension (slacks creek) and beaurepairs and they both try to tell me nothing is wrong with it. drag link, tie rod ends etc etc have been checked and adjusted, its been wheel aligned about 4 times and nothing seems to help.

has anyone got any ideas on what might be wrong?...apparently my tyres arent round anymore and the wheels cant be balanced but i would have thought that would only cause vibration and not make the car wander but am i wrong? would a good quality return-to-centre steering damper help stabilise things, would bigger/smaller tyres help? new shockies (the ones on there i think are getting a little tired)?

i know its never going to drive fantastic because its an old truck but it used to drive alot better than it does now
any advice would be fantastic