car troubles

[3ps14]

I recently purchased a 240sx ka24de off a guy on Zilvia.net... and it worked out fine first day... drove home like nothing... later that night... the car started to bog and die slowly... we jump started it... and it lasted for like 1 minute and then died again... I figured it would be the alternator, so replaced that, and charged the battery... the next day.. same problem happened... lights worked, accessories worked, when I tried to start the engine it turns over, but will not start. Any suggestions?

Much Appreciated..