There are lots of reasons an engine would overheat. Here are some:
Bad radiator cap
Clogged radiator passages
Leaking head gasket
Lean fuel/air mixture
Blocked airflow to/from radiator
Worn out fan clutch
Low oil level
If you know you have a coolant leak, I'd start there. Find it and fix
it. This may be difficult, now that you've put in "stop leak".
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