Opel Vectra B 1.7 TD ISUZU Motor

Good, I have a problem with a vertra B 1.7 TD 96 with Isuzu motor, when he wants (lately almost always), when putting the contact he trembles the velocimeter needle and when you start it does not work the count or velocimetter but the kilometers.
If when you put the contact the needle does not tremble, you start it and everything works well.
I have changed the alternator and the painting and remains the same.
The alternator load is scarce, about 12.5 V, sometimes 13.5 V
thanks in advance