NO DON'T DO IT YOURSELF, TAKE IT TO A TRUSTED MECHANIC. sorry caplocks is a b**ch, but seriously i had the exact same problem.....i was dragging though when mine got to 120 it just cut out and then got exactly the same stuff happen, no speedo, s**t excelleration but goes hard off its nuts after 90-100km/h. pretty sure it didn't cost to much to get fixed either.
anyway get it to a mech and you'll be right

so just take it to him and say i want the speed sender changed/fixed?

Hmmm... ooookaaay....
The speedo senders are less then $10 from Ford, and while they are fiddly to do, it isn't hard.
Ellery's manual says to do it from underneath the car, but unless you have little baby hands it'll probably be a pain in the a***.
I just ripped out the centre console and shifter and did it from above.
Check the 'Speedo Calc' document http://www.fordmods.com/forums/documents.php?doc=30
to find out which sender you need mate.
Even gives you the part number on there!