Low hours/high hours - not sure which is worse.
Had a 2000 Nissan 50 that had seeming overheating issues. Replaced water pump and thermostats,
checked fuel lines/fuel bulb, corrosion/sediment in heads, etc. Ran well much of the time, but this issue would keep returning.
Cannot comment on your spring question directly, as my mechanical skills are relatively non-existent.
Ended up purchasing a Tohatsu 50 4 stroke - works great.