Does good from first to second but sometimes does not shift after

Hi and thanks for using 2CarPros.com. Understand, there can be many causes which result in this type of problem. Do the basics first. Make sure the fluid is clean and full. If you decide the fluid is dirty and should be chanced. Here are general directions:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

If the check engine light is on, you need to identify which codes are stored. Here is a video which shows how to retrieve codes:

https://youtu.be/YV3TRZwer8k

The reason I mention checking codes is because there may be an issue with a shift solenoid in the transmission. If you find a code which correlates with a shift solenoid, there was a technical service bulletin (TSB) which was put out years ago with regards to erratic shifting. Here is the bulletin:

A/T - SHIFT SOLENOID UPDATE
File In Section: 7 Transmission

Bulletin No.: 37-71-48A

Date: November, 1996

INFORMATION

Subject:
Transmission Product Updates
(New Shift Solenoids)
Models:
1991-97 Chevrolet and GMC C/K, P, G Models
1991 Chevrolet and GMC R/V Models with HYDRA-MATIC 4L8O-E Transmission (RPO MT1)
This bulletin is being revised to update the part numbers and to include 1995-97 model years. Please discard Corporate Bulletin Number 377148 (Section 7 - Transmission).

Service Information

Starting August 14, 1996 (Julian date 226) all HYDRA-MATIC 4L80-E transmissions are being built with the new solenoid assemblies. The new solenoids are serviced for all 1991 through 1997 4L80-E applications.

Parts Information

Previous Part New Part Description

10478125 10478132 Valve assembly, 1-2 Shift Solenoid (Black)

10478126 10478133 Valve assembly, 2-3 Shift Solenoid (White)

If you notice, the 2-3 shift solenoid was updated. The shift solenoids are internal in the transmission and are attached the valve body, which is under the transmission pan. NOTE: If you get a trouble code related to the 2-3 shift solenoid, use the color diagram above to find it in the transmission,

I hope this helps. Let me know if you have other questions.

Take care,
Joe