on the aircraft the marginally higher JPT on a 100 series was not visible on this type of gauge. In this day and age you would probably see it on a digital gauge but not on those. This was the reason we were doing daily engine runs to check bleed valve settings in the early eighties when there was an issue with the BVCU diagphram material until they were modified. That was checked on strip according to the report so can't see it would have been the problem. We have come a long way in terms of engine instrumentation nowadays ! If there had been a noticeable lack of thrust the RPM would have been continuously at the top end of the scale to achieve enough thrust you would think , so the thrust reduction doesn't make sense unless there was a transient high RPM/JPT fault which caused him to momentarily throttle back ? We'll never know.