There must be many degrees of freedom with adjusting a garage door, so I thought this would best be done by a professional. Using Angie's List, I picked "Payless Garage Doors" which had done work in our neighborhood and had a good rating. Plus they had a $39 door tune-up and inspection which seemed like what we needed. The door worked okay mostly, it just had a tendency to reverse and not close occasionally.
The "tune-up and inspection" took about an hour. He sprayed silicone lubricate on all the rollers. Most of his time was spent in shortening the cable on one side, to "try to get the door opening evenly". He was using how the door hit the ground as his measure of "evenness". This didn't seem to address the problem I was concerned about -- the door sort of jerked as it came down, rather than moving smoothly. He said this was a property of the dual spring design, and to do better I needed to replace it with a torsion spring. He also said I needed new rollers.
I can believe that we need new rollers. These are the original rollers from when the house was built, so 27 years old. Researching rollers on the internet, it appears there are two kinds: plastic (which are quieter) and metal (which last longer). Also it's best to get them with actual ball bearings in the middle. Home Depot and Lowe's both have a set of two for $4.50, and with shipping, internet suppliers don't do much better. I need 10 rollers -- five sets of 2 -- for each door. I bought them at Lowe's for $24.30 (including tax). Genie brand.
It took a bit over an hour to replace them. There are YouTube videos showing how to replace them by bending the track, replacing the rollers and then bending it back, but I just used a socket wrench to remove each roller bracket, replace the roller and then put it back on. Half I did overhead, standing on a step stool; the other half I did by closing the door and replacing them vertically. It didn't seem to matter much either way.
After replacing the rollers, I'm not sure that it made any difference. The door still seems to work pretty much the same. Maybe the difference is subtle.
Update: the difference is not subtle. Nothing has improved. The work done by Payless Garage Doors did nothing to fix the problem. So I called another company -- Cedar Park Overhead Doors. I put off calling, not expecting them to do any better. But when I finally called to schedule an appointment, they said they could have someone out that afternoon, at 2:00. They were prompt and after hearing the problem and seeing the door open, said the problem was the pulleys -- the pulleys needed to be replaced. I said I could do that myself, and they said "No Problem, I'll just write this up as an estimate -- our estimates are free." Their estimate to replace the pulleys was $80 for the pulleys and about $100 for the labor to put them in. I tried to pay him for the service call, but he refused.
I went to Lowe's and got 4 Genie replacement pulleys -- $20 total. Then it took about an hour to replace the four pulleys. And he was right. The ball bearings in at least one of the pulleys were clearly shot. Replacing the pulleys, and un-doing the work that the first guy had done, put the door back to working just fine.
The door now has new rollers and new pulleys. It already had new springs. Only the actual door remains from the original (and it's on my list to replace with a newer, metal, insulated door one of these days!)
