Issues implementing an up/down counter
The key points are:


Ensure that after gating the new arrangement makes logical sense. ie why should it count DOWN now. If you don't follow the logic of what you think it should do then you can't be too surprised if it disagrees with you  :-)
Always change over in a "neutral" state. Ensure that clock lines or other state relevant lines are in a condition before and after the change that will not cause a transient change of state.




A "working" circuit from here



A marvellous (&amp; free)  tool

Logisim - an educational tool for designing and simulating digital logic circuits

They say:


Logisim is an educational tool for designing and simulating digital logic circuits. With its simple toolbar interface and simulation of circuits as you build them, it is simple enough to facilitate learning the most basic concepts related to logic circuits. With the capacity to build larger circuits from smaller subcircuits, and to draw bundles of wires with a single mouse drag, Logisim can be used (and is used) to design and simulate entire CPUs for educational purposes.