Why does my 2001 Honda Civic keep overheating?

Ok so I'm having many problematic symptoms with my 01 Honda Civic. 1st thing is that my check engine light is on, it's been on for about 8 months now, the code says that it is my vehicle speed sensor, when I took it to the shop they said the the speed sensors were fine and that they're putting out the correct ohms, they suspect it could be a wiring problem, what are your thoughts on that? 2nd my car likes to fluctuate between 2000 and 3000 rpm when I first start to drive but goes away for a little bit, often times when I'm sitting at idle the car once again fluctuates between 1000 and 0 rpm when it hits 0 rpm the car shakes uncontrollably, sometimes my engine would just shut off, suspect it could be a vacuum leak i don't know. 3rd problem, even when I fill the radiator and reserve with coolant, it still overheats after long drives, maybe 45 min or more of highway driving then when I get off it starts to overheat. It never overheats when I go to work which is about 10-15 min away. When ever I open my radiator cap and reserve the next day after it overheats there's hardly anything in there, anyone know why this is happening? How much would it cost to hey all of this fixed?

Your car sounds screwed up. For the check engine light if I were you I'd get a second opinion because for your second problem it sounds like there may be something wrong under the hood that needs to be checked. As far as your coolant check and make sure there aren't any leaks. This all could be very expensive to fix though

You may need an engine. 2001 Civic's were famous for bad blocks and transmissions.

http://www.carcomplaints.com/Honda/Civic/2001/engine/engine_failure.shtml

It could be your Ecu which is faulty, or might need to be reprogrammed to fix the engine light. Over all I'd say try a new engine, maybe you fan is faulty? Could be down to the way you drive it, if you tend to be in a lot of traffic, but still keep the revs high, that can make it heat up. Over all about 800€.

The code your getting could very well be the wiring. Get a second opinion. The idle fluctuation is due to your IAC acting up due to an air bubble in the cooling system created from a leak somewhere in the system. Take your car to a shop and have the system pressure tested before your ruin your engine or blow a head gasket.