running rough while driving down the road

about 2 months ago my 88 XJ started acking funny while going down the freeway it started acking like it was backfiring. i am not sure what it is if it is the motor or the trany. some days it seems like it is fule but the filter has been changed not even 2 months ago. the bigest problem is it is not consistent. i can be on the highway or even the streets it does not matter. and tonight it even died on me at a stop light. it starts right back up but is still was running rough at constant speeds cause if i stand on it it goes away for a few minutes then it will come back.

stock motor and trans in an 88 XJ

Start by testing your sensors. Start with the TPS.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm