I guess the simplest answer is whatever makes them happy. I do very little T&M now, but I did it for years. Some of those older clients prefer T&M and if that makes them happy, it's fine with me. With new client I'd talk about a troubleshooting charge to locate the problem, then bid the repair.
The tough one for me so far is that I can find some problems in 5 minutes, others take much, much longer, so the troubleshooting fee is a guess. You can try to make it up in the repair bid.
Dave
