Vehicle Speed Sensor Problem...Please HELP!!

I am about to give up on my car ever working again...if you have any
idea what the problem with my car might be, please help! Thanks in
advance for all of your input...here is my problem:

I have a 99 Mercury Cougar and this summer, while driving on the
interstate, the speedometer dropped to zero and the transmission would
no longer shift properly...I hooked up a code reader and it told me
that my vehicle speed sensor was malfunctioning...I took my car to a
shop and had them install a new VSS...the car worked fine for a couple
of hours, but then the same problem started happening - the
speedometer would not work (it would only work "sometimes"...very
sporadic) and the car would shift extremely hard...so I decided that
my car was in fact still broken and I took it to another shop in town
(a more reputable one)...after their investigative work, they were not
able to find any problems with the VSS wiring (which I told them to
check out) and they cleaned off all the connections...the only problem
they found was that the VSS was not completely installed by the first
shop I took it to (I guess it was not correctly locked into
place)...so they finished the installation...their only other
suggestion was that it might be some other major electrical problem
with my car involving my car's computer???...anyway, after having the
VSS "correctly" installed, my car now works slightly better, and by
slightly I mean that the speedometer and transmission now consistently
DO NOT WORK the first 10+ minutes of driving, but one the car gets
sufficiently warm (? - just a guess) the speedometer kicks up and then
follows the transmission shifting perfectly...this now happens very
consistently - if I drive my car long enough, everything works fine.

So there is my problem, do any of you have any suggestions on how to
fix this? I have seriously looked at everything I could on the
internet and I don't really have the $$ to take it to the local ford
dealership and give them a blank check to tear my car apart...I am so
frustrated that I would sell my car right now if I could get any money
out of it...thanks again in advance for all of your help!

hey, i was having the EXACT SAME problem... taking the car to several
shops with no sucess..... it did get better for awhile (where it only
did it while the carw as cold) but now it is all the time, i drive
around in 2nd most of the time im in town (at like 3,000 to 4,000 rpm)
just so my transmission wont shift on me.... SOMONE HELP
im gonna try to find the solution myself cause i dont want to take the
damn thing to another garage until i know exactly what the prolem is
and how to fix it. please let me know if you find a solution and i
will let you know if i find one