The tachometer for the MD7A diesel on my Islander 28 is misbehaving. It does not work when the engine is started but 'comes to life' after the engine is run continuously for about 15 min or so. If the engine is stopped with the tach operating correctly and then immediately restarted, the tachometer will again not work for some period of time. After the tachometer begins to operate, it seems to read correctly most of the time but the meter needle will occasionally bounce around and even quit working for a few minutes. Is there some way I can tell if it is the meter or the sender which is at fault? Changing the tach doesn't seem like too much of a job, but has anyone out there replaced the tach sender on this engine who could tell me how difficult this job would be? Also, are these two parts (tach and sender) available somewhere?