What is the refrigerant you're running and what is the ambient temp? If you're on 134a, that wouldn't be too expected on a hot day (90+) due to its relatively poor performance in those conditions. When it's hot outside, 134a needs a higher mass-flow and a decent air flow over the condenser. With the fans you're running, the air-flow is probably ok, but the mass-flow is down due to the lower RPM of the compressor.

Have you thrown a gauge on the system to find out what the pressures are doing? Charge adequate? You could have a sticking expansion valve if it's otherwise been operating correctly, but suddenly doesn't. The high/low pressure differential can tell you a lot.

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