Booting from on-chip memory works, but resets every few seconds

In Qsys, I have the cpu reset and exception pointing to a 32-bit wide 28k onchip ram. The linker setting for the nios app has everything pointing to the same onchip ram.

At the moment, the on chip app code (7k code+initialized data, 21k stack/heap) that's synthesized into the logic design, consists only of:

// Init System

alt_irq_init (ALT_IRQ_BASE);

ALTERA_AVALON_TIMER_INIT ( TIMER1_SYSTEM_500US, Timer1_System_500us);

ALTERA_AVALON_TIMER_INIT ( TIMER2_GENERAL_1MS, Timer2_General_1ms);

ALTERA_AVALON_SYSID_QSYS_INIT ( SYSID, sysid);

// Init UART

IOWR_ALTERA_AVALON_UART_CONTROL(UART_DEBUG_BASE, 0x0000); // inhibit all UART IRQ sources

//alt_ic_isr_register(UART_DEBUG_IRQ_INTERRUPT_CONTROLLER_ID, UART_DEBUG_IRQ, DBG_ISR, &context_dbg_uart, 0x00 );

IOWR_ALTERA_AVALON_UART_DIVISOR(UART_DEBUG_BASE, (ALT_CPU_FREQ/DEFAULT_BAUD_RATE_115200) + 1);

// Reset Status/Error Flags

IOWR_ALTERA_AVALON_UART_STATUS(UART_DEBUG_BASE, 0);

IOWR_ALTERA_AVALON_UART_CONTROL(UART_DEBUG_BASE, ALTERA_AVALON_UART_CONTROL_RRDY_MSK );

IOWR_ALTERA_AVALON_UART_TXDATA ( UART_DEBUG_BASE , '1' );

while ( 1 )

{

while ( 0 == ( ALTERA_AVALON_UART_STATUS_TRDY_MSK & IORD_ALTERA_AVALON_UART_STATUS ( UART_DEBUG_BASE ) ));

IOWR_ALTERA_AVALON_UART_TXDATA ( UART_DEBUG_BASE , '2' );

usleep ( 1000000 );

}

When I power-on, I see the '1' across the RS-232, and then the '2', but a second later, I see the '1' again, and the reset led indicator toggles. I'm thinking some (instruction?) exception is being thrown, or perhaps some kind of stack overflow corruption is going on, but I can't find anything unusual.. nor any way to pinpoint what exactly is causing the reset.

Any ideas?