We had some debris hanging off the bottom of our door that was breaking the IR beam and making the door stop and reverse because it thought something was in the way.
You might try disconnecting the door from the motorized track and try moving the door manually and see how smooth it operates. Sometimes the rollers get out of alignment a little and cause enough resistance that the motor stops and reverses. And sometimes the tracks will shift over time, causing the same result.
A good spring test (while you have it disconnected from the track) is to lift the door about half way and let it go. It should pretty much stay and not drift down.
A visual check of the springs should be done. You'd be looking for gaps in the springs, meaning they've been over stressed and can no longer hold the load safely. Spring and cable kits can be bought at any big discount store and are usually sold by color on the end of the spring, indicating how much weight they will sustain. I bought mine off ebay and they've been working fine. I didn't know what color I needed so on advice from a garage door tech, I took a bathroom scale and put it under the disconnected door and weighed it to get a ballpark weight. Seemed to work.
When we moved into our house we replaced both garage doors. Took us a couple days but it wasnt' very hard at all. Just want to study the present track setup and make sure you're new tracks will work. If you're just replacing the springs and motor, it's pretty simple. But try to make sure you run the safety cables through the springs so if the springs break the cable will stop them from shooting across the garage or, worse, injuring someone or putting a dent in your vehicle(s). At an apartment we lived in years ago one of the springs let go in the middle of the night and the apartment mgt. didn't put the cables in. Sounded like a gun shot. Went to the garage and found the spring in the corner after putting a good sized dent in the side of our new-ish Thunderbird. Management fixed the spring and paid for getting the car fixed.
