Just purchased a 99 Impreza with problem.When I start it starts right up,
runs 15sec+/- then dies.If I open throttle slow it dies imediately.If I rapidly
work throttle it will keep running,but if I hold throttle at 1 point no matter rpm
it dies in a few seconds.Fuel pres. is 45 steady even after it dies.

Disconnect the MAF and try again. If nothing changes, bet on your MAF being dead. Unfortunately, we've got very wimply MAF sensors in the 99s.

look into the 05rs!!!

just a thought, might be the O2 sensor going bad...had kinda the same problem on my '00 RS, replaced the sensor and ran fine till it burnt out again. I'm thinking its the high-flo Cat. on mine.