Not an engineer

Hi I'm new to mixing, but generally enjoying using MixPad. However I have an issue which seems to occur after I duplicate some tracks (to pan one left and the other right) - it may be a stupid thing to do, but with the accompaniment central and voices either side sounds good to me! Anyway, the playback stutters and jumps about, making it sound like a broken record. When I export to MP3 the exported file is fine, but I need to be able to adjust levels before exporting, and this is driving me crazy. It's not just this one file, and not just today. Any suggestions??