Trim Slipping at higher power setting

[P.Grow]

New Cub owner - C85/0-200. When cruising at speeds of anything over 80mph, I try to trim the nose to come up, it slips. I turn the crank but no movement on the indicator.
If I slow to anything just below 80mph, I can trim with no difficulty. Local mechanic says this is common with J3's and he could tighten the cable but it may not solve the problem. Assures me the screwjack mechanism is fine.

Insights?

 

[dnnewman]

 

[dnnewman]

 

[huston marlowe]

 

[huston marlowe]

 

[reileyr]

 

[bob turner]

 

[dnnewman]

 

[jimrice]

 

[Chris Maher]