j30 alternator problem or what????

1995 infiniti j30 (bought it june 2004 used......no problems until now)

Drove around 10-15 miles on highway, stopped in city. As I was pulling out of my parking spot I noticed the dash lights dim and the car suddenly died.

Got a jump start and after a minute I pulled into drive and the car died again.
Got another jumpstart and left it running idle for 20-30 minutes until my friend could drive by and escort me home.

I was able to get home (around 5-10 minute city driving), parked and turned off the car. I tested to see if I could start it again, but it failed to start.

MEchanic came and put in a new battery and alternator, but after testing with some voltage meter he finds the car is just running off the battery......
He put in another alternator with the same results.
He tested spark plugs and all are working....

Any ideas on this problem?

THANKS