I have 2 140s, and have had this same issue on one. First, I run on kero, not diesel - this is pump kero and is filtered. On the turbine with the same issue, abruptly dying at 55 - 65 rpms, I went in and relearned RC and set both the acceleration and deceleration settings from fast to normal. After this change, the turbine ran with no issues. You may want to try that and see if it helps.
One other thing to note, the outing when the problem started, the weather was hot and humid, where previously (with no issues) the weather was much cooler.
Robert