Like others have said between the slanging about "proper aircraft"...(whatever they are)
An APU genny can be the starter as well.
The new geneation 737's ie -800 have a starter / generator.
An IDG is an Integrated Drive and genny in one housing.
Another way of doing it was a genny mounted on a CSD constant speed drive unit.Genny could usually be changed without the CSD but IDG changed as one unit.
On APU's, genny speed is generally controlled by APU speed which is usually constant or near to that, ie 99 to 101%.
So if a genny on an APU has frequency issues quite possible its a fuel control problem.On an main engine more likely a CSD or IDG issue as frequency is controlled by some very clever devices as engine speed varies widely.
Mostly generic comments but I'm sure you get the idea.