Hi, Marco;
there are mainly two types of Spices (the HP3x series) concerning their internal structure. One type is solderless, and all components connect with each other through a flex circuit that is actually 'pressed' against the IC's, LED display assembly and power board terminals. The behavior you describe is somehow common to this type of Spices, namely bad contact. The other one uses a rigid mainboard and all IC's, LED assembly and power board connectors are soldered.
Considering that the serial number of these calculators may be changed once they may have they back case replaced at any moment, I see no way to distinguish one type from the other by visual inspection. Two references may be added, though.
The solderless units are heavier because they have a fair piece of metal that ensures all contacts are made by offering a plane, uniform surface that regularly distributes the pressure to all terminals and contacts of the flex circuit. A solderless unit without batteries weights approximately the same as a soldered unit with their batteries inside. The other reference also needs comparison, though. The solderless units have a single flex circuit that also has molded keyboard contacts, while the units with soldered IC's have metal caps. As a consequence, when pressing the keys of the solderless unit it's needed a wider rotating angle to actually close their corresponding contact (they go deepre inside the calculator's case), while the keys of the soldered units need to move only about half the `path` to actually close their contacts as well.
The best way is having one of each in hands so you can feel for yourself.
Once you find out that your unit is one of the solderless type, DO NOT THROW IT AWAY! There are directions on how to open them at the MoHPC Articles Forum so you can open it and clean the contacts.
Good luck and success.
Luiz (Brazil)