Some bard songs not stacking after instrument equipped
 

Some bard songs are not lasting the full effect from equipping an instrument -- for instance, I should be able to equip my lute one time and after the enemy is hit, I can take it off and should do full dmg per tick. It works fine with the lvl 18 dot, but just not chords and perhaps other spells too (haven't tested).

For instance,

*casts chords with lute*
Enemy winces..
12 dmg..
*takes off lute*
6 dmg.... (should continue for 12 to utilize the old strat to switch instruments when casting each dot)

It works fine currently, you're doing something wrong. You're supposed to have a lute equipped on the first application of the song. After that, if you unequip your lute, you need to stop playing the song (or play a different song) or else it will reapply without an instrument modifier.