It sounds like your mechanics are not very knowledgeable or skilled.
Start by having the problem properly diagnosed, not guessed at.

I don't have a suggestion for you on this problem, but I think what Steve
meant was to take your car for a diagnosis at a shop that really understands
these things.  From the sounds of it, your mechanic really does not and it
quickly gets expensive if you keep throwing parts at problems.  This is a
great forum to bring these types of question to and often enough you can
find the answers right here... but those typically are for the more well
defined problems.  So many of these problems today are only well defined
through the connection of the scan tool.

It's not my experience that any of these types of  problems (similar to
yours) have ever been related to the ignition switch, so that one really has
me scratching my head.  I'd be more inclined to believe a wheel hub is
beginning to fail.  The ABS sensor is located inside of/and integral part of
the hub and bearing assembly.  A scan tool will reveal which of the hubs is
reporting a failure.  Without that, you've got 4 hubs to chose from... pick
one.  I sure would not be surprised if a 2000 Cavalier needed a hub at this
point.

It's often best to take Steve's advice and get the car to a reputable
mechanic or the dealer and scan the car.  At least you'll know what to fix.

Well I can almost guarantee you that anyone who has had these symptoms will
be able to tell you what's wrong with your car. It would be about the same
as someone telling you to replace your O2 sensor if you MIL was on because
that's what happened to them. "My check engine light is on, what's wrong."

If you are getting two mechanics telling you two different things, then at
least one (maybe both) didn't even look at your car. Or if you are getting
one mechanic telling you it's one thing, then something else, get a second
opinion. Posts like yours pop up on here day after day. "This guy told me it
could be this, this guy told me it could be that." If you took it to a
mechanic who diagnosed it and told you what the problem was you wouldn't
even be here posting this nonsense.

There is nothing anyone in here can do for you other than tell you to take
it to a mechanic, unless you are of above average intelligence and know how
to diagnose the vehicle yourself, but then you wouldn't be in here asking
questions no one can answer. ;)

Steve

Hehe. Like it or lump it.

So anyway, if you still want help, here's some more sarcastic remarks:

You are leaving out a lot of information, it's hard for someone on the other
end of a computer to fill holes in stories.

You say the mechanic told you it was a speed sensor so you replaced it. Did
the mechanic tell you which one? (don't get pissy, it happens) Did the
mechanic "diagnose" it, or did he stick a scan tool on it and say "ya, it's
this sensor." See, if all he did was stick a scan tool on it and say the
right front sensor is open or shorted, replacing it will not necessarily fix
the problem, it could be the wiring.

I believe Ian was just snipping at someone a few threads back about taking
the car into the dealership and having it diagnosed but only to take it
home, replace the part himself. Which didn't fix the problem and now they
were here complaining about it.

You never posted what the trouble codes were, in a case like this, it a
necessity when looking for help here. It probably is a common problem and
someone here probably knows the answer, but how would you know, all you
basically told us is "TCS, ABS, MIL on, mechanic told me it was speed
sensor, I replaced it, didn't fix the problem, now he tells me it's the
ignition switch." Fill in all the holes and the answer will come.

Steve