Re: help. vibration above 70km
It is possible that the torque converter is out of balance, it is not unheard of. Also, it may be loose. You can check that by removing the splash plate in front of the gearbox and checking the bolts.
If the car has been lowered, it is possible to get driveline vibes, because the tailshaft angle can be changed outside its optimum operating angle.
If the car isn't lowered, I would firstly undo the tailshaft and turn it 180 deg, and bolt it back up to see if that makes any difference. Secondly, I would then check all uni joints. Take the shaft off when you do this. See if there is ANY lateral movement in the joints, and also check for roughness in the bearings. You can feel the roughness be moving the joints carefully.
If any of the above exist, replace the joints, and do them both, 'cos if one is going, the other is not far behind.
Another thing I would do is check the shaft for straightness. The car may have had a whack up the bum and bent the shaft.
As far as oil pressure goes, no it is not usual for that to happen. It sounds like your pressure relief valve may be stuck. As gordon451 says: rig up an analogue guage and check. It's not really difficult, you just put a "T" piece where the oil pressure switch is, and connect the original guage and run off an analogue guage.