Hi, My name is ***** ***** for the delay.
I will do my best to assist you
if that 4l60e transmission is shifting as if it gos into neutral when it is time to shift to third,the problem is internal.
the 3-4 clutches are burning out.
i would also think that soon it will also loose the 4th gear.
there is no easy fix for this.
the transmission will need to be rebuilt or replaced,sorry
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