Here's a new improved patch to try:

Index: psycho.c
===================================================================
RCS file: /cvsroot/syssrc/sys/arch/sparc64/dev/psycho.c,v
retrieving revision 1.39
diff -p -u -r1.39 psycho.c
--- psycho.c    2001/10/07 20:30:41     1.39
+++ psycho.c    2001/12/13 19:21:20
@@ -416,6 +416,7 @@ found:
         * arrive here, start up the IOMMU and get a config space tag.
         */
        if (osc == NULL) {
+               uint64_t timeo;
 
                /*
                 * Establish handlers for interesting interrupts....
@@ -444,6 +445,28 @@ found:
                psycho_set_intr(sc, 1, psycho_wakeup,
                        &sc->sc_regs->pwrmgt_int_map, 
                        &sc->sc_regs->pwrmgt_clr_int);
+
+
+               /*
+                * Apparently the kernel is spending too much time in nucleus
+                * mode with interrupts disabled and a number of machines with
+                * psycho and psycho+ controllers are losing interrupts.
+                * We'll try setting the interrupt retry timeout to 0xff
+                * (which is what sysio is set to) for the moment, which seems
+                * to help alleviate this problem until the trap handlers can
+                * be revamped.  
+                */
+               timeo = bus_space_read_8(sc->sc_bustag,
+                       (bus_space_handle_t)
+                       (u_long)&sc->sc_regs->intr_retry_timer, 0);
+               if (timeo < 0xff) {
+                       printf("increasing interrupt retry timeout "
+                               "from %lx to 0xff\n", (long)timeo);
+                       bus_space_write_8(sc->sc_bustag,
+                               (bus_space_handle_t)
+                               (u_long)&sc->sc_regs->intr_retry_timer, 0,
+                               0xff);
+               }
 
                /*
                 * Setup IOMMU and PCI configuration if we're the first