diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/SpectrumBase.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/SpectrumBase.cs
index 43c0ec8b7a..11ec6ae96b 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/SpectrumBase.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/SpectrumBase.cs
@@ -123,7 +123,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
 
             PollInput();
             
-            while (CurrentFrameCycle <= ULADevice.FrameLength) // UlaFrameCycleCount)
+            while (CurrentFrameCycle < ULADevice.FrameLength) // UlaFrameCycleCount)
             {
                 // check for interrupt
                 ULADevice.CheckForInterrupt(CurrentFrameCycle);
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULABase.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULABase.cs
index d6e3b1ef66..1604006525 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULABase.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ULABase.cs
@@ -254,7 +254,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
         /// <summary>
         /// The longest instruction cycle count
         /// </summary>
-        protected const int LONGEST_OP_CYCLES = 23;
+        protected int LongestOperationCycles = 23;
 
         /// <summary>
         /// Signs that an interrupt has been raised in this frame.
@@ -288,19 +288,19 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
                 return;
             }
 
-            if (currentCycle < InterruptPeriod)
+            if (currentCycle < LongestOperationCycles)// InterruptPeriod)
             {
                 // interrupt does not need to be raised yet
                 return;
             }
 
-            if (currentCycle > InterruptPeriod + LONGEST_OP_CYCLES)
+            if (currentCycle >= InterruptPeriod + LongestOperationCycles)
             {
                 // interrupt should have already been raised and the cpu may or
                 // may not have caught it. The time has passed so revoke the signal
                 InterruptRevoked = true;
                 _machine.CPU.FlagI = false;
-                
+                return;
             }
 
             if (InterruptRaised)
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.ULA.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.ULA.cs
index d7cc8f4941..92789fa82c 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.ULA.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum128K/ZX128.ULA.cs
@@ -9,6 +9,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             : base(machine)
         {
             InterruptPeriod = 36;
+            LongestOperationCycles = 23;
             FrameLength = 70908;
             ClockSpeed = 3546900;
 
diff --git a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.ULA.cs b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.ULA.cs
index a093a33aad..f72bc8bde1 100644
--- a/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.ULA.cs
+++ b/BizHawk.Emulation.Cores/Computers/SinclairSpectrum/Machine/ZXSpectrum48K/ZX48.ULA.cs
@@ -9,6 +9,7 @@ namespace BizHawk.Emulation.Cores.Computers.SinclairSpectrum
             : base(machine)
         {
             InterruptPeriod = 32;
+            LongestOperationCycles = 23;
             FrameLength = 69888;
             ClockSpeed = 3500000;