When you slow the plane down to a stall, does it always drop the same wing? Two things to check are wing twist and lateral balance.

First check the wing for twist with an incidence meter. If the wing is twisted it can usually be corrected by twisting the wing by hand until the covering wrinkles, then reshrink with your heat gun while holding the twist. It may take several tries. It's surprising how much twist you can put into or take out of a wing this way. It will make a huge difference in the stall characteristics.

Once you have the wing incidence trued test for lateral balance. Trim for level flight then make some long, straight inverted passes. Does it want to roll? Even a little? If so then it needs balancing. If it rolls left when inverted, add a little weight to the left wing tip and try again. A plane that is out of lateral balance will have aileron trim to correct it. This effectively gives more angle of attack on the side with the drooped aileron and that side will tend to stall first.

Let us know what you find.

Dave