Could be tilt (adjust directly, and if you don't have a micro-adjustable seatpost, you can shim with a little piece of metal cut out of a can with snips). Could be reach to the handlebars (shorter stem and/or steeper rise). Could be saddle shape - too wedge-shaped forcing you forward as you pedal.

My bet is on trying a shorter or steeper stem, since it's worse when you sit more upright. The other two are likely to be worse when you're in a deeper tuck, the opposite of what you're experiencing.