tyrax,
the UE can come from 3G to 2G in idle mode, in speech dedicated mode or/and in packet dedicated mode.
All scenarios are possible, and might be independantly activated or deqctivated.
If you have an issue of idle pingpong between 3G and 2G, then you must ensure that idle “exit parameters” from 3G have weaker values than the idle “exit parameters” from 2G.
You have to put both idle algo side by side in order to find a stable stragegy.
Regards
pix