4L60E constant shifting between 3rd and 4th

[yukonrrican]

Hi to all:
I have a 1997 Yukon GT 4wd with 124K miles. It has 3.46 diff with 33in tires. This weekend driving in the highway it start to shift constantly between 3rd and 4th shift between 50 to 60 MPH. This morning driving to my job, it don't do it at all.

Any advice will be appreciated.

 

[BoxerPitMix]

I had a similar problem except it was my torque converter locking/unlocking like crazy. Turned out to be a bad valve body.

 

[98_k1500]

Jiggle the brake pedal a little bit, the brake switch can cause this.

 

[Purefection]

Brake switch or torque converter. Common problem on those trucks is the brake switch. Jiggle the pedal around while it's doing the switching and see if it stops. Also check your tranny fluid.