Check Engine light issue code P0750

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My check engine light is coming on with a code P0750 Shift solenoidA Malfunction. I have had the light turned off about 4-5 times at a local Advance Auto store. This code has been appearing over a period of about 4 months now. I have been told all kinds of stuff about it and a little leary about who is telling the truth. I have been told that the part cost $85 from one mechanic and $500 from another mechanic. I am on disability and live on a very limited income and can't afford to overpay or pay for dishonest work. The Jeep is shifting fine except when the light comes on and then a couple times when you give it the gas it does not go with full power. The RPM's go up but with little power to the drivetrain. Each time that has happened I used some Lucas transmission fix addative. That has happened twice and once I used a quart of regular fluid. I have not noticed any leaks on the ground since the problem has started. But the fluid was low 3 times and I added a quart each time. So that has been 3 quarts added. The transmission was serviced about 9 months ago with a fluid and filter change. The Jeep presently has 160,000 miles. It has not been driven rough or harshly with most miles being highway. Most recently, when the check engine light came on and the fluid was checked it was full. And the drivetrain was responding with little or no power. It was immediately parked and later checked and the light turned off at another Advance Auto and it was shifting and doing fine. This issue is just about enough to drive you crazy and you do not feel comfortable driving anywhere. I am alway afraid of breaking down and an expensive bill. I really can't afford to just go and buy another auto. I need my transportation for groceries and doctors. Thankyou in advance for any honest advice.