So instead of bidding on your project, i will just give you some suggestions for things to look for. Some are very
obvious and simple while others might be a little more non-obvious.
1. First, is this problem limited to only ONE part or are several affected? In other words, the part may be defective.
2. Check the VCC line to see that you aren't applying too much voltage (this particular HC part is rated for 10V max).
3. Make sure that the Analog Inputs =never= exceed the VCC supply voltage. If they do, the analog signals can possibly
"bleed" into other input channels causing something like what you describe.
4. Make sure that the VCC and GND wiring is a very SOLID low impedance path (not, for example, tiny little wires).
5. Check that the VCC and GND lines are quiet and noise free and that there is a 0.1uF decoupling capacitor near to the part.
This is more important in TTL Logic circuits but is still helpful to eliminate stray noise even in CMOS logic circuits.
6. Verify that the channel select control lines are clean and that none of them are "floating" and make very certain that
none of the select lines are shorted to each other. That would also cause some of the things you described (the problem
may then also be in the circuit that is driving those lines).
7. Verify that the Enable pin is solidly grounded when the part is in operation.