You can classify the contact on the nav map on all the submarines. Just right click on the contact.
Are you getting the wild speed estimates on DEMON or on TMA? Your Auto TMA will give you crazy data sometimes since it doesn't "talk" to sonar, it only works on bearing information. If its on DEMON make sure the cursor is on the far left of the first line (the "Shaft Line" I think that one is called). Also count the blades (should be 7) to make sure its the right contact, you can merge two separate contacts in to one by mistake.
__________________
 
|