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The contacts could be starting to arc weld together. Contacts in an import starter are tiny compared to NEMA contacts. The inrush current of a 5 HP motor is significant.
Disconnecting contacts actually arc more than when the contacts slam together. Asian import starters are usually cheaper than a set of contacts for a NEMA grade starter so if things don't work out and you're considering shiny from China then keep the predicted durability in mind. Good NEMA starters that will hold up with that motor will be above $400.00 new. You can pick up a used one on eBay for significantly less but those starters will likely require some homework before buying.