For some odd reason while standing outside and.

For some odd reason while standing outside and car was left running after driving 25 miles to my destination, the engine started revving up and down constantly. The fan came on and was quite noticeable. After awhile the engine overheated and anti freeze running onto the ground. I was completely stunned. Let it cool down and put water in it. Drove back home just in time before it started to overflow again. At this time temp gauge does not work and lights going dim. At night. Car would not start when I made it home. Problem continued every time I got into van. Engine revving up and down and overheating. Again would not start. Had van towed and was told alternator no good and battery tested ok. Eventually I got alternator fixed but could not use van until I did. The revving and stalling of the van stopped, ran better but still overheating with gauge still not working. When I asked that question here on this site back when I was told alternator would fix all these problems but not so. Heats good inside van. Will this be a problem with sensor controlling water flow due to engine temperature? I don't see leaks on pavement before overheating. Water pump? Radiator plugged? Thermostat? CAN YOU HELP ME. Very frustrated. Put 6-7000 $ in car and only paid 6000 when I bought it 3 years ago. My e-mail address is wolverineman69@hotmail. Com.