01 Mazda Millenia 2.3l Driving 1/2 Mile Rpm Guage Starts Dropping

I have a 2001 Mazda Millenia which was dying on my wife while she was driving. She would drive about 1/2 a the car will quit. I had the battery tested and the battery was bad so I purchased a new one. I purchased a new one and the car runs fine at idle but when I leave my driveway and go about 1/2 mile and then the rpm guage begins to drop. I did not notice a deceleration of speed. The RPMs went from about 1200 and I drove 5 miles to my mechanic and watched the RPMs drop to about 250 RPMs. Car began to die but I had turned it off before noticing any of the light or guage readings or if the engine light would blink. Do you know what would cause the car to RPMs on the guage and die. Mechanic can't find anything wrong but says it could be the alternator but wants to charge $250.00 plus tax to change it.