Driver side under steering wheel next to hood release

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I have a 2001 Toyota Tacoma Prerunner 2WD 5VZ V6 engine. Got the taco with a little under 100k. 1 time owner and was well kept. No history of transmission though.Checked fluid periodically. Seemed okay so didnt think to change fluid. So now at 120k it was slowly having harsh shifting within the third gears and jerking forward at a stop. Also metal pinging noise when first shifting in drive or reverse. So i did drain and refill the fluid. Did not drop the pan. Got 3 quarts out of the drain plug tilted on driveway. Fluid was dirty brown but not black and no burn smell. Manual says 1.7 for that enigine for a drain and refill which seemed quite low. I put 2 quarts. Drove a little and checked dipstick constantly. It seemed a little low so i put half quart in. Shifted better for a few days. No jerking at a stop and shifted smoother. Then it started acting up again so i drained and refilled 2 more times. Still the same but off and on. Runs great on highway just bad at stop and go. Mainy when hot and been driving over 30 mins or so. Please help! Im racking my brain on this. Could it hopefully be a solenoid or something better than a transmission major repair i hope???Any advice would be grateful. Thanks What seems to make the problem better or worse? Adding more fluid or changing fluid How long have you had this problem? 1 month