diff --git a/Source/ePC-8801MA/vm/event.cpp b/Source/ePC-8801MA/vm/event.cpp
index 547551a..bbfddd9 100644
--- a/Source/ePC-8801MA/vm/event.cpp
+++ b/Source/ePC-8801MA/vm/event.cpp
@@ -17,6 +17,12 @@
 #ifdef SDL
 #define SINGLE_EXEC_TIMEOUT		10000
 										// exit single exec mode (us)
+#define SINGLE_EXEC_TIMEOUT_8MHZ	20000
+										// 8MHz mode needs wider window for stable FDC timing
+#define SINGLE_EXEC_SLICE		4
+										// max clocks per single-exec step
+#define SINGLE_EXEC_SLICE_8MHZ		20
+										// 8MHz mode needs larger slice for CPU to keep up
 #endif // SDL
 
 void EVENT::initialize()
@@ -145,8 +151,9 @@ void EVENT::drive()
 {
 #ifdef SDL
 	if (single_exec == true) {
-		// if passed 10ms, disable single_exec
-		if (passed_usec(single_exec_clock) > SINGLE_EXEC_TIMEOUT) {
+		// if passed timeout, disable single_exec
+		int timeout = (config.cpu_type == 0) ? SINGLE_EXEC_TIMEOUT_8MHZ : SINGLE_EXEC_TIMEOUT;
+		if (passed_usec(single_exec_clock) > timeout) {
 			single_exec = false;
 		}
 	}
@@ -244,8 +251,9 @@ void EVENT::drive()
 			if (main_cpu_exec > 0) {
 				// single execution ?
 				if (single_exec == true) {
-					if (main_cpu_exec > 4) {
-						main_cpu_exec = 4;
+					int slice = (config.cpu_type == 0) ? SINGLE_EXEC_SLICE_8MHZ : SINGLE_EXEC_SLICE;
+					if (main_cpu_exec > slice) {
+						main_cpu_exec = slice;
 					}
 				}
 				cpu_done = d_cpu[0].device->run(main_cpu_exec);
@@ -263,8 +271,9 @@ void EVENT::drive()
 				if (cpu_remain > 0) {
 					sub_cpu_exec = cpu_remain;
 					if (single_exec == true) {
-						if (sub_cpu_exec > 4) {
-							sub_cpu_exec = 4;
+						int slice = (config.cpu_type == 0) ? SINGLE_EXEC_SLICE_8MHZ : SINGLE_EXEC_SLICE;
+						if (sub_cpu_exec > slice) {
+							sub_cpu_exec = slice;
 						}
 					}
 
@@ -277,8 +286,9 @@ void EVENT::drive()
 				if (cpu_remain > 2) {
 					sub_cpu_exec = cpu_remain / 2;
 					if (single_exec == true) {
-						if (sub_cpu_exec > 4) {
-							sub_cpu_exec = 4;
+						int slice = (config.cpu_type == 0) ? SINGLE_EXEC_SLICE_8MHZ : SINGLE_EXEC_SLICE;
+						if (sub_cpu_exec > slice) {
+							sub_cpu_exec = slice;
 						}
 					}
 					
diff --git a/Source/ePC-8801MA/vm/upd765a.cpp b/Source/ePC-8801MA/vm/upd765a.cpp
index 8811cf0..1273a9c 100644
--- a/Source/ePC-8801MA/vm/upd765a.cpp
+++ b/Source/ePC-8801MA/vm/upd765a.cpp
@@ -400,7 +400,9 @@ void UPD765A::write_signal(int id, uint32 data, uint32 mask)
 	} else if(id == SIG_UPD765A_TC) {
 #ifdef SDL
 		// phase==PHASE_EXEC support (for Xanadu Scenario II)
-		if(phase == PHASE_READ || phase == PHASE_WRITE || phase == PHASE_SCAN || (phase == PHASE_RESULT && count == 7) || (phase == PHASE_EXEC)) {
+		// but disable for 8MHz mode where it destabilizes Wizardry boot
+		bool tc_accept_exec = (config.cpu_type != 0);
+		if(phase == PHASE_READ || phase == PHASE_WRITE || phase == PHASE_SCAN || (phase == PHASE_RESULT && count == 7) || (tc_accept_exec && phase == PHASE_EXEC)) {
 #else
 		if(phase == PHASE_READ || phase == PHASE_WRITE || phase == PHASE_SCAN || (phase == PHASE_RESULT && count == 7)) {
 #endif // SDL
@@ -518,7 +520,7 @@ void UPD765A::set_drq(bool val)
 		if((command & 0x1f) != 0x0d) {
 #ifdef SDL
 			// for customized event manager + 2HD disk
-			register_event(this, EVENT_LOST, 30000, false, &lost_id);
+			register_event(this, EVENT_LOST, 15000, false, &lost_id);
 #else
 			register_event(this, EVENT_LOST, disk[hdu & DRIVE_MASK]->get_usec_per_bytes(1), false, &lost_id);
 #endif // SDL
@@ -684,8 +686,8 @@ void UPD765A::cmd_recalib()
 
 void UPD765A::seek(int drv, int trk)
 {
-	// get distance
-	int seektime = 32 - 2 * step_rate_time;
+	// get distance (step rate is in msec units, convert to usec)
+	int seektime = (32 - 2 * step_rate_time) * 1000; // msec -> usec
 	if(disk[drv]->drive_type == DRIVE_TYPE_2HD) {
 		seektime /= 2;
 	}
@@ -1466,7 +1468,15 @@ double UPD765A::get_usec_to_exec_phase()
 	int drv = hdu & DRIVE_MASK;
 	int trk = fdc[drv].track;
 	int side = (hdu >> 2) & 1;
-	
+
+#ifdef SDL
+	// Use constant exec-phase delay for SDL mode.
+	// The SDL event system's position tracking is not precise enough
+	// for rotational-position-based timing, which causes timing-sensitive
+	// titles (e.g. Wizardry) to fail during boot.
+	return 100;
+#endif
+
 	// XXX: this is a standard image and skew may be incorrect
 	if(disk[drv]->is_standard_image) {
 		return 100;