Sabre, first of all, you have the splitter after the cable box, right? Just making certain. I'd guess the cable company would tell you that the box won't put out a signal strong enough to be split, so you'll "need" a second box. However, I've never seen a box that couldn't have its output split at least once.

The output of the box would be on channel 3 or 4, and not change as you switch channels, so the tuned-in cable channel shouldn't affect the signal strength. You should have plenty of signal, or weak signal, accross all of the channels. The signal strength should depend on the output of the RF modulator at the box output.

To answer your question, an RF booster (amp) shouldn't help, meaning that it shouldn't be necessary, but if it does help, there might be something wrong with the box. Again, the same boost should be needed accross all channels, or none.

The only thing that makes sense to me is that for some reason, only digital channels are affecting the output strength, but why only on some channels? Are the channel numbers you mentioned the only high numbers you get, or are they the only ones that drop out?

Last thought: try swapping the output channel from 3 to 4, or 4 to 3, see if that makes any difference. Otherwise, ask the cable company to let you try another box. It makes no sense that you'd go from great signal to dropping out, and nothing between. "Dropping out" is a digital phenomenon; analog usually weakens 'smoothly'.

Larry
www.fineelectricco.com