If you have not already done so, might be worth running over all the running gear with a spanner, It is not unusual for things to loosen off in time...
The first thing I would be looking at would be play in any of the wheels, with the wheels on the deck, grab the top of a front wheel and give it a good shake if you can, the weight of the car can help load up any possible play to make it`s self heard or felt. Jacked up and if you grab both the front and rear of a wheel and pull and push with left then right hands, again you might pick up on any play in the system..which as has been suggested can e down to steering gear wear, causing tracking to be out by some degree..
Try the simple stuff first, best of luck.
