just swap a sohc into my 89 coupe ran good for a day and now doesnt

[1991black240sx]

the other day i had to swap a new motor into my 89 coupe. i bought the car with a blown sohc and i took the one thats in it now from my 91 hatch that had an sohc. It ran good for the first day that i drove it. whe i woke up yesterday morning to go to drill my battery was dead, i jumped the car and started to drill. i only made it about 15 miles and everytime i step on the gas the car sputters and if i hold it down long enough the instrument panel, headlights, and radio start to shut down. when it does this the motor will rev up and the car is slowing down. it seems that the transmission is slipping into neutral. its an automatic. i'm not sure what to check first. now i cant seem to get it to run longer than a minute. it needs to be jump started and it just sputters out. please help.

[sx moneypit]

First thing i would do is check the alternator output.

[1991black240sx]

i did some looking around on it last night and found that it was the alternator output was not fully tightened. i went to tighten it and snapped the stud off of the alternator. thankfully i have the entire motor that was in it and i took the alternator off of it and put it on the car. i jump started the car and had no problems from before. runs like a champ now. thank you.

[sx moneypit]

No problem brother!