Just a couple of questions to start the invest going...
1. Do the revs stay there at 3 to 4k until you pull away and drop on acceleration?
2. Do they hunt up and down between 3 and 4?
3. How about when you first start the car, do the revs do the same when the engine is cold?
4. And finally does it start at any particular time like when the engine is warm or after a period of driving?
Hopefully we can get to the bottom of this soon bud. My automatic Mondeo used to change gears from 4th to 1st whilst at 70mph on the M'way. Some scary poop when it happens. But sounds like your issue can be fixable.
fastjetfixer everything you said is exactly whats happening but it happens when the engine is cold or hot. It doesnt drop gears yet but this is what im worried that might happen next, also the automatic transmission comes up on the dashboard occassionally,and that incident with you on the motorway is something i wouldnt like to go through.
Sounds like the throttle body and/or Idle speed control unit are gunked up, the gasket connecting it to the maifold maybe perished or the breather hose(s) may be split/perished. Try the following and see how you go. If you have a Haynes manual you're laughing. The following might differ slightly as your model is not only auto but a different year to mine. Just a tip but I usually take photos as I am going along so I can put it all back together the right way.
1. Take the air box off by using a 5mm(?) allen key. Sometimes the heads
are chewed up on these bolts and you need to cut a groove in them to
get a big screwdriver in instead. That can be a pain. You may as well
take the whole inlet trunk off as one. There is a pipe on the underside
of the right hand side of the air box that leads to the Idle Control
Valve. Take this pipe off too by undoing the jubilee clip. Old type
original GM jubilee clips are 1/4" whereas the new ones if they have
been replaced are 7mm.
2. Reach your arm around the side of the engine following where the pipe
you just disconnected goes to. You should feel the valve and be able to
see it if you look hard enough. You need to unplug the ECU connector
from it by squeezing the edges.
3. There is a jubilee clip that holds it onto the inlet manifold, you
need to undo this. This is the trickiest part - If the person who last
cleaned it did it from above they will have left the jubilee clip facing
your way so you can get your 1/4" or 7mm socket with extension onto it
and undo, or a screwdriver if you find it easier.
4. Once undone pull it out from underneath the manifold and spray the
inside of it with injection intake cleaner. I use the Wynns stuff, about
a fiver from Halfords. Spray copious amounts in and move the valve with
a screwdriver or similar. The cleaner fluid that comes out should be a
5. Give it a spray of WD40 to prevent sticking. Not really needed but just in case.
6. Place it back under the manifold remembering where it was located
exactly. The small black rubber pipe needs to be pushed back on (check it is still nice and flexible with no splits/holes). Easier said than done. Then another fiddly bit - Getting the jubilee
clip tightened from a remote location.