Plymouth Neon Engine Runs Rough

I Have A 2000 Neon That Backfires At Low Speeed Under A Load.at High Speed Runs Great.at Idol Rpm's Fluctuate And Sometimes Race.the Engine...

My 1998 Plymouth Neon Runs Good Until The Motor Warms Up Usually After You've Driven About 4 Miles, Then It Starts Running Very Rough...