SOME THINGS TO LOOK FOR IF YOU HAVE A STEERING WOBBLE
Check the bearings in the neck. Make sure the steering bearings are tight but not so tight as to make it hard to turn it right or left.
Make sure your tires are not out of round, tire pressure correct (you can adjust the pressure up/down to see how it affects the handling.
Make sure your trail is within limits, not more than 6" less is better.
Make sure your rear brakes are not "dragging and the drum or rotor is not out of round.
Wheel base too short.
Fork sliders worn.
Fork Flex. (Maybe you can add a front fork brace)
I'm sure there are more causes and if you can add to this list please email me and let me know so that I can add it here.
Most VW Trikes will have a pesky low speed wobble. You can add a 63 - 79 VW Bus steering stabilizer to get rid of it.