diff --git a/source/CPU.cpp b/source/CPU.cpp
index 46050a4a..93e1289f 100644
--- a/source/CPU.cpp
+++ b/source/CPU.cpp
@@ -427,6 +427,13 @@ static __forceinline void CheckInterruptSources(ULONG uExecutedCycles)
 	}
 }
 
+// GH#605: IRQ needs to occur within 17 cycles (6 opcodes) of configuring the timer interrupt
+void CpuAdjustIrqCheck(UINT uCyclesUntilInterrupt)
+{
+	if (uCyclesUntilInterrupt < IRQ_CHECK_TIMEOUT)
+		g_nIrqCheckTimeout = uCyclesUntilInterrupt;
+}
+
 //===========================================================================
 
 #include "CPU/cpu6502.h"  // MOS 6502
diff --git a/source/CPU.h b/source/CPU.h
index 1c86461f..98f58d0a 100644
--- a/source/CPU.h
+++ b/source/CPU.h
@@ -14,6 +14,7 @@ struct regsrec
 extern regsrec    regs;
 extern unsigned __int64 g_nCumulativeCycles;
 
+void    CpuAdjustIrqCheck(UINT uCyclesUntilInterrupt);
 void    CpuDestroy ();
 void    CpuCalcCycles(ULONG nExecutedCycles);
 DWORD   CpuExecute(const DWORD uCycles, const bool bVideoUpdate);
diff --git a/source/Mockingboard.cpp b/source/Mockingboard.cpp
index a52fd4e2..f9bf7aed 100644
--- a/source/Mockingboard.cpp
+++ b/source/Mockingboard.cpp
@@ -427,6 +427,7 @@ static void SY6522_Write(BYTE nDevice, BYTE nReg, BYTE nValue)
 			pMB->sy6522.TIMER1_COUNTER.w = pMB->sy6522.TIMER1_LATCH.w;
 
 			StartTimer1(pMB);
+			CpuAdjustIrqCheck(pMB->sy6522.TIMER1_LATCH.w);	// Sync IRQ check timeout with 6522 counter underflow - GH#608
 			break;
 		case 0x07:	// TIMER1H_LATCH
 			// Clear Timer1 Interrupt Flag.
@@ -444,6 +445,7 @@ static void SY6522_Write(BYTE nDevice, BYTE nReg, BYTE nValue)
 			pMB->sy6522.TIMER2_COUNTER.w = pMB->sy6522.TIMER2_LATCH.w;
 
 			StartTimer2(pMB);
+			CpuAdjustIrqCheck(pMB->sy6522.TIMER1_LATCH.w);	// Sync IRQ check timeout with 6522 counter underflow - GH#608
 			break;
 		case 0x0a:	// SERIAL_SHIFT
 			break;