diff --git a/libdevcore/Worker.cpp b/libdevcore/Worker.cpp
index c4ea4996b41..089d783caa5 100644
--- a/libdevcore/Worker.cpp
+++ b/libdevcore/Worker.cpp
@@ -30,15 +30,17 @@ using namespace dev;
 void Worker::startWorking()
 {
 //	cnote << "startWorking for thread" << m_name;
-	Guard l(x_work);
+	std::unique_lock<std::mutex> l(x_work);
 	if (m_work)
 	{
 		WorkerState ex = WorkerState::Stopped;
 		m_state.compare_exchange_strong(ex, WorkerState::Starting);
+		m_state_notifier.notify_all();
 	}
 	else
 	{
 		m_state = WorkerState::Starting;
+		m_state_notifier.notify_all();
 		m_work.reset(new thread([&]()
 		{
 			setThreadName(m_name.c_str());
@@ -49,6 +51,7 @@ void Worker::startWorking()
 				bool ok = m_state.compare_exchange_strong(ex, WorkerState::Started);
 //				cnote << "Trying to set Started: Thread was" << (unsigned)ex << "; " << ok;
 				(void)ok;
+				m_state_notifier.notify_all();
 
 				try
 				{
@@ -68,47 +71,55 @@ void Worker::startWorking()
 //				cnote << "State: Stopped: Thread was" << (unsigned)ex;
 				if (ex == WorkerState::Killing || ex == WorkerState::Starting)
 					m_state.exchange(ex);
-
+				m_state_notifier.notify_all();
 //				cnote << "Waiting until not Stopped...";
-				DEV_TIMED_ABOVE("Worker stopping", 100)
-					while (m_state == WorkerState::Stopped)
-						this_thread::sleep_for(chrono::milliseconds(20));
+
+				{
+					std::unique_lock<std::mutex> l(x_work);
+					DEV_TIMED_ABOVE("Worker stopping", 100)
+						while (m_state == WorkerState::Stopped)
+							m_state_notifier.wait(l);
+				}
 			}
 		}));
 //		cnote << "Spawning" << m_name;
 	}
+
 	DEV_TIMED_ABOVE("Start worker", 100)
 		while (m_state == WorkerState::Starting)
-			this_thread::sleep_for(chrono::microseconds(20));
+			m_state_notifier.wait(l);
 }
 
 void Worker::stopWorking()
 {
-	DEV_GUARDED(x_work)
-		if (m_work)
-		{
-			WorkerState ex = WorkerState::Started;
-			m_state.compare_exchange_strong(ex, WorkerState::Stopping);
+	std::unique_lock<Mutex> l(x_work);
+	if (m_work)
+	{
+		WorkerState ex = WorkerState::Started;
+		m_state.compare_exchange_strong(ex, WorkerState::Stopping);
+		m_state_notifier.notify_all();
 
-			DEV_TIMED_ABOVE("Stop worker", 100)
-				while (m_state != WorkerState::Stopped)
-					this_thread::sleep_for(chrono::microseconds(20));
-		}
+		DEV_TIMED_ABOVE("Stop worker", 100)
+			while (m_state != WorkerState::Stopped)
+				m_state_notifier.wait(l); // but yes who can wake this up, when the mutex is taken.
+	}
 }
 
 void Worker::terminate()
 {
 //	cnote << "stopWorking for thread" << m_name;
-	DEV_GUARDED(x_work)
-		if (m_work)
-		{
-			m_state.exchange(WorkerState::Killing);
-
-			DEV_TIMED_ABOVE("Terminate worker", 100)
-				m_work->join();
-
-			m_work.reset();
-		}
+	std::unique_lock<Mutex> l(x_work);
+	if (m_work)
+	{
+		m_state.exchange(WorkerState::Killing);
+		l.unlock();
+		m_state_notifier.notify_all();
+		DEV_TIMED_ABOVE("Terminate worker", 100)
+			m_work->join();
+
+		l.lock();
+		m_work.reset();
+	}
 }
 
 void Worker::workLoop()
diff --git a/libdevcore/Worker.h b/libdevcore/Worker.h
index 23c90ab788a..22dd88743f6 100644
--- a/libdevcore/Worker.h
+++ b/libdevcore/Worker.h
@@ -99,8 +99,9 @@ class Worker
 
 	unsigned m_idleWaitMs = 0;
 	
-	mutable Mutex x_work;						///< Lock for the network existance.
+	mutable Mutex x_work;						///< Lock for the network existance and m_state_notifier.
 	std::unique_ptr<std::thread> m_work;		///< The network thread.
+    mutable std::condition_variable m_state_notifier; //< Notification when m_state changes.
 	std::atomic<WorkerState> m_state = {WorkerState::Starting};
 };
 
diff --git a/libethashseal/Ethash.cpp b/libethashseal/Ethash.cpp
index 17cd9e8a3c9..53321588e19 100644
--- a/libethashseal/Ethash.cpp
+++ b/libethashseal/Ethash.cpp
@@ -60,6 +60,12 @@ Ethash::Ethash()
 	});
 }
 
+Ethash::~Ethash()
+{
+	// onSolutionFound closure sometimes has references to destroyed members.
+	m_farm.onSolutionFound({});
+}
+
 strings Ethash::sealers() const
 {
 	return {"cpu"};
diff --git a/libethashseal/Ethash.h b/libethashseal/Ethash.h
index 9763fb6dd26..ef1bc26ea21 100644
--- a/libethashseal/Ethash.h
+++ b/libethashseal/Ethash.h
@@ -37,6 +37,7 @@ class Ethash: public SealEngineBase
 {
 public:
 	Ethash();
+	~Ethash();
 
 	std::string name() const override { return "Ethash"; }
 	unsigned revision() const override { return 1; }