Guido, are you sure that your data is STORED as Numeric(18,5)? I've read that if you upgrade from IB5.x to dialect 3, data of type Numeric(whatever) don't "upgrade" their storege type (double in this case). You have to "rebuild" the db and reimport the data. Could it be the case?
If not, could you provide us a SQL script with a simple table, filled (insert...) with some data, so I can easely test your problem by myself? I'm a little afraid by what you are experiencing...
Regards
Marco Menardi