My previous experience with Alphas I always ran cooler at idle and just slightly hotter at high speed RPM. It sounds, to me, like you don't have enough flow at idle. When you last replaced your impeller, did you just replace the impeller itself or did you use the Merc "kit" which includes the plastic housings? Also I would suggest a new water pump (on the engine)

3 things I have found in my travels, 1 already discussed is thermostat & debris from previous impeller failure. 2 I found this a few times the impeller not bonding to the shaft sleeve, (shaft turns normally but impeller slipping on sleeve). 3 Exhaust risers plugged up with rust not allowing water to exit properly,just a couple of ideas. Randy

the stainless impellar housing could be scarred up as well