I think at this time of year when you're going to be chilly on the flat but too warm on the hills you've got to kinda stick it out.
How long are the climbs? Are we talking 2-3 mins, half an hour or longer . ( BTW we only have very short but steep little climbs where I live so I can get very hot very quickly, then back to being a bit chilly).

If you can unzip and ventilate as much as possible as you can on the climbs, you can then wrap up again for the decents/flat sections.

Do you wear any head-gear under you helmet? You could may be leave that behind at the moment or remove it for the steep climbs.

I find this time of year is a real balancing act between too cold and too hot. Might just have to experiment a bit till you find the right combination.

PS - I find things like arm warmers and wind proof gillets are really usefull, so you can add & take away from your outfit as and when. Hope this helps