My 2004 elantra speeds up and slows down when driving. it does do it as hard when you go over 65 mph.

Sounds like the speedometer is faulty. If the speed sensor were the problem, the odometer would not work correctly, the transmission would not shift correctly, and the check engine light would be on.

This could be the brake pads dragging or it could be a wheel bearing. You can check one or the other. The wheel bearing is tested by jacking up the car with that wheel off the ground and see if you can rock the wheel side to side or up and down. The way to check the brakes is to take the wheel off and look at the brake pads.

I do not know if this vehicle has a torque converter lockup or not, but sounds like it. Does the transmission stop shaking when you go over 65? Some vehicles have a torque converter lockup, which means when the vehicle is traveling at a certain speed in high gear the torque converter "locks" and there is no slippage to the transmission resulting in better fuel economy. if the torque converter doesn't "unlock" when you slow down to say 45 then the engine will "lug" causing vibration. If this is the case you are not hurting anything by driving, just shift to 3 when it shakes then upshift under lighter load, or take it to a shop that replaces transmission sensors.

I am thinking that the car was telling you that traction was at a minimum at the speed you were travelling at in those conditions and advised you to slow down and when you slowed down you were back in the safety zone.

could be tires need balancing or front end alinement

Find a shop with a 'rolling road' or 'electric chassis ears'.

Chassis ears are microphones you place under or around the car and drive it listening to each microphone to locate the noise.

Rolling road you can listen whilst going nowhere.

Sound like you have a collapsed engine mount allowing engine to rub somewhere.

Shop search near you : http://www.iatn.net/shopfinder/

You remove and clean the valve body

No luck,you rebuild the unit

I used to own a trans shop

sounds like a warped rotor to me

time for a meshanic to take a look

No....the rpms differ from engine to engine and from manufacturer to manufacturer. RPM's are always higher at higher speeds.

You sould have recieved a "recall" notice in the mail about the cruise control for fire and failure issues. they are adding a fusable link where the cruise control harness connects to the brake master cylinder. if you have not had the recall performed yet you should SERIOUSLY consider getting it done, mainly so it doesnt catch fire, but "could" be the result of the failure you've been experiancing! btw, almost ALL Ford vehicles have been affected with the cruise control recall. hope this helps!