problem with unsigned long variables

Hello,
After two years of happy working with the keil C166/C167 compiler I've just ran into a problem I don't understand / can't solve.

Running phytec kitcon 167
Memory model Hlarge

Declared two unsigned long variables :

unsigned long v1;
unsigned long v2;

problem part of code :

if (v1!=0)
  {
    v2 := v1;
    .....
    .....
  }

At regular intervals but only once in
200 .. 1000 times the code loses the two
least significant bytes op v1.
e.a.
v1 : 0x0001 5BC9
normal outcome , v2 : 0x0001 5BC9
false outcome , v2 : 0x0001 0000

memory location of v1 : 0x0004 1750
memory location of v2 : 0x0004 1776

Behavior in simulator looks like behaviour on the board.

I've looked at the disassembly :

MOV   R6,DPP2:0x1750
MOV   R7,DPP2:0x1752
MOV   R4,R6
OR    R4,R7
JMPR  CC_Z,0x003A3C
MOV   DPP2,0x1776,R6
MOV   DPP2,0x1778,R7

for

if (v1!=0)

the compiler stores value in r6 and r7
When error occurs r6 is good in line

if (v1!=0)

but is cleared ! before line

v2:=v1

is executed.
r6 was holding two least significant bytes.

Can anybody help ?

Best regards,
S.Nijssen
Motoplex