Injected database context fails to connect at high load - entity-framework

Question

I am using AutoFac for dependency injector in web api project.
I am resolving dependency like below...
builder.RegisterType<DbContext>().InstancePerRequest().WithParameter("nameOrConnectionString", "name=TestDbContext");
builder.RegisterType<TestDbContext>().InstancePerRequest();
builder.RegisterType<TestDbContext>().As<ITestDbContext>().InstancePerRequest();
builder.RegisterType<TestDbRepository>().As<ITestDbRepository>().InstancePerRequest();
It is working fine when hit a single api route. But when our QA team doing performance testing with jMeter which hitting multiple request at a time, some of requests get fail and get db related error, like below errors...
The underlying provider failed on Open.
System.InvalidOperationException: The connection was not closed. The connection's current state is connecting.
at System.Data.ProviderBase.DbConnectionClosedConnecting.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
at System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1 retry)
at System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry)
at System.Data.SqlClient.SqlConnection.Open()
at System.Data.Entity.Infrastructure.Interception.InternalDispatcher`1.Dispatch[TTarget,TInterceptionContext](TTarget target, Action`2 operation, TInterceptionContext interceptionContext, Action`3 executing, Action`3 executed)
at System.Data.Entity.Infrastructure.Interception.DbConnectionDispatcher.Open(DbConnection connection, DbInterceptionContext interceptionContext)
at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.<>c__DisplayClass1.<Execute>b__0()
at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func`1 operation)
at System.Data.Entity.Core.EntityClient.EntityConnection.Open()
But this error is not consistent some times it get passed but some times this error comes.
Please help me if any thing we need to modify with Autofac dependency resolver.