1) The CD4000 series is about the slowest logic family that's still on the market. Switching to anything else - 74LS, 74HC, take your pick - will improve your fMax dramatically.
2) Every IC on your board needs a decoupling capacitor as close to its power/ground pins as possible. Not having those is probably causing a lot of weird behavior, including the "interference" you're seeing from other devices.
1) The CD4000 series is about the slowest logic family that's still on the market. Switching to anything else - 74LS, 74HC, take your pick - will improve your fMax dramatically.
2) Every IC on your board needs a decoupling capacitor as close to its power/ground pins as possible. Not having those is probably causing a lot of weird behavior, including the "interference" you're seeing from other devices.