diff --git a/libethereum/BlockChainSync.cpp b/libethereum/BlockChainSync.cpp
index c8c81c37f..bd0b10c7c 100644
--- a/libethereum/BlockChainSync.cpp
+++ b/libethereum/BlockChainSync.cpp
@@ -364,7 +364,6 @@ void PV60Sync::transition(EthereumPeer* _peer, SyncState _s, bool _force, bool _
 {
 	clog(NetMessageSummary) << "Transition!" << EthereumHost::stateName(_s) << "from" << EthereumHost::stateName(m_state) << ", " << (isSyncing(_peer) ? "syncing" : "holding") << (needsSyncing(_peer) ? "& needed" : "");
 
-	//DEV_INVARIANT_CHECK;
 	if (m_state == SyncState::Idle && _s != SyncState::Idle)
 		_peer->m_requireTransactions = true;
 
diff --git a/libethereum/BlockQueue.cpp b/libethereum/BlockQueue.cpp
index 4555ecb2b..254bf3300 100644
--- a/libethereum/BlockQueue.cpp
+++ b/libethereum/BlockQueue.cpp
@@ -37,7 +37,7 @@ const char* BlockQueueChannel::name() { return EthOrange "[]>"; }
 const char* BlockQueueChannel::name() { return EthOrange "▣┅▶"; }
 #endif
 
-size_t const c_maxKnownCount = 100000;
+size_t const c_maxKnownCount = 10000;
 size_t const c_maxKnownSize = 128 * 1024 * 1024;
 size_t const c_maxUnknownCount = 100000;
 size_t const c_maxUnknownSize = 512 * 1024 * 1024; // Block size can be ~50kb
@@ -196,6 +196,7 @@ ImportResult BlockQueue::import(bytesConstRef _block, BlockChain const& _bc, boo
 
 	// VERIFY: populates from the block and checks the block is internally coherent.
 	BlockInfo bi;
+
 	try
 	{
 		// TODO: quick verify
@@ -435,34 +436,35 @@ bool BlockQueue::unknownFull() const
 
 void BlockQueue::drain(VerifiedBlocks& o_out, unsigned _max)
 {
-	WriteGuard l(m_lock);
-	DEV_INVARIANT_CHECK;
-
-	if (m_drainingSet.empty())
+	bool wasFull = false;
+	DEV_WRITE_GUARDED(m_lock)
 	{
-		bool wasFull = knownFull();
-		m_drainingDifficulty = 0;
-		DEV_GUARDED(m_verification)
-		{
-			o_out.resize(min<unsigned>(_max, m_verified.size()));
-			for (unsigned i = 0; i < o_out.size(); ++i)
-				swap(o_out[i], m_verified[i]);
-			m_verified.erase(m_verified.begin(), advanced(m_verified.begin(), o_out.size()));
-		}
-		for (auto const& bs: o_out)
+		DEV_INVARIANT_CHECK;
+		wasFull = knownFull();
+		if (m_drainingSet.empty())
 		{
-			// TODO: @optimise use map<h256, bytes> rather than vector<bytes> & set<h256>.
-			auto h = bs.verified.info.hash();
-			m_drainingSet.insert(h);
-			m_drainingDifficulty += bs.verified.info.difficulty;
-			m_readySet.erase(h);
-			m_knownSize -= bs.verified.block.size();
-			m_knownCount--;
+			m_drainingDifficulty = 0;
+			DEV_GUARDED(m_verification)
+			{
+				o_out.resize(min<unsigned>(_max, m_verified.size()));
+				for (unsigned i = 0; i < o_out.size(); ++i)
+					swap(o_out[i], m_verified[i]);
+				m_verified.erase(m_verified.begin(), advanced(m_verified.begin(), o_out.size()));
+			}
+			for (auto const& bs: o_out)
+			{
+				// TODO: @optimise use map<h256, bytes> rather than vector<bytes> & set<h256>.
+				auto h = bs.verified.info.hash();
+				m_drainingSet.insert(h);
+				m_drainingDifficulty += bs.verified.info.difficulty;
+				m_readySet.erase(h);
+				m_knownSize -= bs.verified.block.size();
+				m_knownCount--;
+			}
 		}
-		if (wasFull && !knownFull())
-			m_onRoomAvailable();
 	}
-
+	if (wasFull && !knownFull())
+		m_onRoomAvailable();
 }
 
 bool BlockQueue::invariants() const