diff --git a/VAX/vax_cpu.c b/VAX/vax_cpu.c
index b68cc72e..10b9e700 100644
--- a/VAX/vax_cpu.c
+++ b/VAX/vax_cpu.c
@@ -1635,13 +1635,16 @@ for ( ;; ) {
     case TSTL:
         CC_IIZZ_L (op0);                                /* set cc's */
         if ((cc == CC_Z) &&                             /* zero result and */
-            ((((cpu_idle_mask & VAX_IDLE_ULTOLD) &&     /* running Old Ultrix or friends? */
+            ((PC - fault_PC) == 6) &&                   /* 6 byte instruction? */
+            (fault_PC & 0x80000000) &&                  /* in system space? */
+            (((cpu_idle_mask & VAX_IDLE_VMS) &&         /* VMS 5.0 and 5.1 */
+              (PSL_GETIPL (PSL) == 0x3) &&              /*  at IPL 3 */
+              (PSL_IS & PSL)) ||                        /*  on the interrupt stack */
+             ((((cpu_idle_mask & VAX_IDLE_ULTOLD) &&    /* running Old Ultrix or friends? */
                (PSL_GETIPL (PSL) == 0x1)) ||            /*  at IPL 1? */
               ((cpu_idle_mask & VAX_IDLE_QUAD) &&       /* running Quasijarus or friends? */
                (PSL_GETIPL (PSL) == 0x0))) &&           /*  at IPL 0? */
-             (fault_PC & 0x80000000) &&                 /* in system space? */
-             ((PC - fault_PC) == 6) &&                  /* 6 byte instruction? */
-             ((fault_PC & 0x7fffffff) < 0x4000)))       /* in low system space? */
+              ((fault_PC & 0x7fffffff) < 0x4000))))     /* in low system space? */
             cpu_idle();                                 /* idle loop */
         break;