Struggle how? Describe the difference in behavior between upright and inverted, please.
I'd be kind of surprised if Rogiero* Fiorotti made the timer so that it would work differently upright and inverted. It's physically possible, though (in fact, you have to specifically make the software so it works the same in both attitudes). He'd have put a menu entry in there that's specific to that, and whatever the effect is, he'd have made it so that the timer is shipped with that feature turned off (if I put such a thing into my timer, it'd be behind a gate that asked "are you sure you want to do this?").
Are you sure it's the timer, and not the trim of the plane? Have you run the thing at a constant speed in the wind and looked for differences upright and inverted? If the plane has issues in one attitude but no the other, and you're running constant speed, that's an issue you should trim out of the airplane first, and then turn the speed regulation back on.
* Apologies on what I am sure is a misspelling.