92 Mitsubishi Galant Auto trans problem

The 92 Galant i own refuses to shift properly. Sometime it shifts properly and while driving the vehicle stalls and becomes really slow. Initially i thought the problem was contaminated oil that might clog the valves inside the transmission. So i asked the mechanic to flush the trans twice and replace the oil and filter. For a few days it worked fine and it is giving trouble again. I dont hear any metallic sound, which might indicate failure of some mechanical part. So i guess the problem could be hydraulic or electrical. Any suggestion on how to fix it would be helpful. Thank you.

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