"SPAD" Simple Plastic Aircraft Design

[Celwin]

I can't see how the aerofoil could affect such a sensitive trim change.
My first thoughts were the C of G is too far back, but last time it flew it slowed down to a crawl and didn't drop a wing proving the C of G is fine where it is. A symmetrical wing tends to have a nicer stall characteristic and isn't as sensitive to changes in pitch, meaning a higher angle of attack before stalling and/or dropping a wing.
It might make a more sluggish affect on your pitch, which is what you're after.
It would also be worth bringing some lead to put in the nose to make certain it isn't a C of G thing.