John,
The RNLA CGT suffers from the same problem: severe shimmy above 25 mph. In between The Final Push liberation tours I checked for any visible play in the steering components. We found play on the drag link and adjusted it, which helped. The shimmy was not gone, but played up at higher speeds and a lot less severe.
Like the others have said, check for play and make sure the axle and steering alignment - wheel camber, axle caster and wheel toe-in - are up to spec. Do you have a maintenance manual? If not let me know, I will scan the relevant pages for you.
If it turns out the pivot pin bearings are worn out: they are available (contrary to popular belief).
HTH,
Hanno
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Regards,
Hanno
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