if all wiring and all connections check out to be ok, then most likely the tuner inside the radio is bad. when it plays on the presets you have on the radio try move it to another. i.e if your station on 1 is cutting out on fm1 try station 1 on fm2 to see if problem goes away. it definally sound like the radio unit is bad.
It is the amp, Remove the glovebox insert and you will see it on the passenger side. It isn't the headunit, While you are in there check all the connections first, It wouldn't hurt to pull the head and look behind there.
Doesn't make sense for it to be the amp if the tape and cd work fine, It sounds like it's the tuner <_<
Amp, Channels, Remove it and you will see where the circuit boards are darkened <_< The headunits on these cars are almost NEVER a problem, But don't discount it, The amps are 98.9% the culprit. Either way you have to take everything out and test it ;)