diff --git a/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/MxGatewayDataConnectionTests.cs b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/MxGatewayDataConnectionTests.cs
index a6548105..b8db9d87 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/MxGatewayDataConnectionTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/MxGatewayDataConnectionTests.cs
@@ -5,6 +5,12 @@ using ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Adapters;
 
 namespace ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests.Adapters;
 
+// Non-parallel with the rest of the DataConnectionManagerActor collection: these adapter
+// tests pump a fire-and-forget event loop (ConnectAsync → Task.Run(RunEventLoopAsync)) whose
+// thread-pool continuation must run before the WaitUntil attach-barrier observes
+// fake.OnUpdate. Serializing within the assembly trims in-assembly thread-pool contention so
+// that barrier (and the negative Unsubscribe assertion that follows it) is not starved.
+[Collection("DataConnectionManagerActor")]
 public class MxGatewayDataConnectionTests
 {
     private static MxGatewayDataConnection NewAdapter(FakeMxGatewayClient fake) =>
@@ -249,7 +255,17 @@ public class MxGatewayDataConnectionTests
             () => adapter.BrowseChildrenAsync(null));
     }
 
-    private static async Task WaitUntil(Func<bool> condition, int timeoutMs = 2000)
+    // Generous ceiling (30 s, matching the DataConnectionManagerActorCollection rationale):
+    // every caller polls a *monotonic, positive* barrier — fake.OnUpdate becoming non-null
+    // (the fire-and-forget event-loop task has attached) or a Disconnected count reaching its
+    // target. Both only ever transition once and never reset, so a longer ceiling merely
+    // tolerates thread-pool starvation under full-solution CPU oversubscription; it cannot
+    // produce a false pass. In Unsubscribe_stops_routing_updates this barrier establishes the
+    // happens-before that the loop is attached BEFORE the test removes the subscription; the
+    // negative assertion (hits == 0) runs afterwards and is independent of this timeout —
+    // the update is delivered synchronously by the test thread, so widening here neither
+    // weakens nor races the negative check.
+    private static async Task WaitUntil(Func<bool> condition, int timeoutMs = 30_000)
     {
         var sw = System.Diagnostics.Stopwatch.StartNew();
         while (!condition() && sw.ElapsedMilliseconds < timeoutMs)