Is your application using Connection Pooling ? Try to check these counters too ..

SQLServer:Memory Manager: Total Server Memory (KB)

SQLServer:Memory Manager: Target Server Memory (KB)

Checkpoint pages per second

Lazy writes / sec

Memory Object: Pages/Sec

PhysicalDisk\Avg. Disk Sec/Read

PhysicalDisk\Avg. Disk Sec/Write

Did you capture any problematic queries that run during that time ? How abt your daily Maintenance Plans ? Are they running successfully ..

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SQLBuddy