1.6 Throttle bodies and spark timing

[ChrisM41]

How stiff are they meant to be? The one on my car is quite light, easily moved and it seem like there is a little slack in the plate before the gearing moves. I got another from eBay but its much stiffer and doesn't have any slack. It doesn't return to the closed position on its own though, and the car didn't like it at all. Thankfully I can return it, but it didn't help with my diagnosis. My cars hesitation/unevenness is mainly under 2000rpm and at light throttle, so was wondering if the throttle body was at fault.

 

Another thing i noticed today is that if I try to hold the car at about 1500rpm (and the throttle is steady as measured by the position % via OBD) the revs go up and down about 50 or so rhythmical, the spark advance/timing advance changes as well (quite a lot, like 50 degrees) in the same up-down rhythm, about once ever 1.5 seconds, any ideas what that might be?