Why does my X-26 Sound Cut out?

[Evitucci]

I have an old POS Fostex X-26, acctually its in really good shape. However when I record two tracks at once. A drum track and a guitar track. I get a jump in the tape right after the drum intro when the guitar comes in. Just like a quick but noticable pause, and the more tracks I add the long ther pause becomes. I have cleaned the heads but this has not helped. And advice would be great.

Thanks,

E