The flanging sound probably means that somehow you are mixing the dry output with the input. Don't know how, but it is pretty recognizeable.
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Ehm, can you explain me this thing better? If it's pretty recognizeable maybe I can solve it..
Not sure that I can, but I'll try. In Pre/Post, one output channel carries the effect, the other carries a dry signal (unless KILLDRY). This signal will be delayed about a millisecond from its input (due to A/D and D/A transition times).
If this delayed "dry" signal is mixed with either input, you will get a "comb-filtered" sound, much like a flanger (this is how a flanger works). You can tell whether it is the effect input or the through input by seeing whether it has the effect on it. If you mix it with the through input you may get some feedback, as you describe.
Now over to you to see how they are getting mixed.
