Comprehensive Bug & UX Optimisation

I need help breathing new life into an existing product that is suffering on three fronts: it loads slowly, the interface feels clunky, and several core features occasionally break or behave unpredictably... (Budget: $250 - $750 USD, Jobs: Debugging, Performance Tuning, Software Testing, User Experience Research, User Interface / IA, Web Application Audit, Web Design, Web Development)