Make sure your monitor supports either Pal or NTSC in composite mode and run the Amiga in the corresponding screenmode. I know the 1084 series was capable of doing 50-60 hz. Most other monitors can't.
I had similar issues with the 20" Toshiba Timm monitor during the 90's. That monitor only supported 60hz ntsc composite modes.
Is your Dell a UK release (pal) or US a model (ntsc)? I suspect if you ordered it from the US it may only support NTSC.
