Tascam Porta02 Questions

[aloicious4]

Hey everyone. I'm new here. Just bought my very first 4-track recorder, the Tascam Porta02, and am having a few problems. Mainly, when I record, no matter what level I record at, when I playback the sound will cut out frequently. So when I'm trying to record a second track, I can't hear hardly any of the first track in order to sync everything up. Am I just recording too high and not know it? Or, since I'm using headphones with a stereo ADAPTER, is that the problem? But when I have it hooked up through my stereo, it still does the same thing. And whenever I try and listen to two or more tracks together, and turn them up in the mix, I get this rhythmic thumping sound instead of the music. And that's whenever I turn the track level up beyond 9 o'clock (since it starts at 7, that's not very much). So I'm just wondering if getting actual stereo headphones will solve my problem, or if I need to record at a much lower level than I am. I downloaded the manual, and it says to record at 0 on the meter (that's the 1st red light). Maybe that's too much for this machine. It was used. Is this a common problem, or did I get a bum unit? Thanks for your help
Sky