My 1995 b14 200sx se (1.6) is having the following problem:

After the car runs for 15 minutes, the heater fan slows down,
the wipers slow down, the dash lights get dimm.
Then while driving, when I press on the brake, the
air bag light starts to flash and the spedometer falls to zero!
When I park the car, the power locks are to weak to lock the car.

Could this be a sign of an alternator problem or
something more serious?

i would think alternator

sounds like the problem

check the voltage on it before you go and change it,