Possbile bug with C51 compiler for T89C51RD2 target

Hi,

I have a strange problem with some code, and I am beginning to suspect that there may be a problem with the C51 compiler or at least my use of it.

The processor runs on a small custom made board that is used to output data via a phy module over UDP to a PC unit. There is no operating system, just the usual while(1) loop. The Atmel has a single watchdog timer that resets the processor if the timer is not reset within a given time.

For some reason, that I have not been able to determine yet, the Atmel processor hangs after approx. 3 minutes running sending UDP packets as it should - it is then eventually reset by the timer. Although I cannot explain why the system hangs, I have however been able to fix the problem by inresting 3 No-operation calls (NOPs) in the code (see code excerpt below). The system will thereafter run for >1 week without failing.

This strange thing is that the code where the NOPs are inserted is never executed, so timing should not be the issue - only memory allocation. Unfortunately the optimization is set to level8. This makes it hard to get an overview.

The one thing I have notiiced in the memory map is that the sections:
?CO?MAIN
and
?PR?TIMER0_ISR?

get switched when adding the NOP functions. The same switch occurs when setting optimization to level=0 (although I can't run this unlinked code).

-Is there anyone out there that has some suggestions as to how I can go about debugging this problem?
-Are there tools available that would give me some more insight into what is going on in memory runtime?
-Any know bug with the compiler that may be the cause (also considering the high optimization level)?

CODE:

void timer0_ISR (void) interrupt 1
{

ET0 = 0; // disable Timer 0 Interrupts

if ((timer0OverflowCount) == 1*HeartBeatTimeOutSeconds)
//(1.02s to be precise)
{
    timer0OverflowCount = 0;
    if (P1_2 == 1)
   //Channel is on. No need to reset if it is off.
   {
     PutString("timer0 timed out because
     of missing heartbeats. Reset imminent.");
     PutStringLn("");
     ResetFlag = 1;
     while(1);
   }
}

 timer0OverflowCount++;   // increase the overflow cnt
        TH0=0;          //reset timer
        TL0=0;
        ET0 = 1;          // Enable Timer 0 Interrupts

        _nop_ ();
        _nop_ ();
        _nop_ ();

}

.

Setup:

Keil uVision2
Compiler: C51&A51 V7.03 (also tried upgrade to V7.08)
Linker V5.02

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Device: T89C51RD2
8051 based CMOS controller with PCA, Dual DPTR, WDT
40 MHz High-Speed Architecture, X2 function, 32/48 I/O lines,
3 Timers/Counters, 7 Interrupts/4 priority levels 64K FLASH,
2K EEPROM (Boot Flash) , 256 Bytes on-chip RAM, additional 1K XRAM
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