A couple of guesses here:

Declare your vars in the interrupt as 'static'. This might fix your problem directly. 'Static' allocates memory for the variable rather than using the variable pool.

The other item which I doubt is causing your problem is when you reload the timer do it as soon as possible in your isr - this is so you don't introduce variable times due to the different execution time of your code - The timer can't re-interrupt since you have yet to do a RETI.

#define TIMER_RELOAD 0xfffd //set timer for 10mS

//Timer 0 Interrupt manages a virtual timer table
void timer0 (void) interrupt 1 using 3
{
static data unsigned char *ptabella;
static data unsigned char i;

TR0 = 0;
//TL0 = (unsigned char)LO_T0;
//TH0 = (unsigned char)HI_T0;
TL0 = TIMER_RELOAD & 0x00ff;
TH0 = TIMER_RELOAD >> 8; //a slightly different of handling the constant!
TR0 = 1;

ptabella = (unsigned char data*) &timer10ms;
for( i = 0; i < LENTIMER10MS; i++ )
{
if( (*ptabella) !=0 )
(*ptabella)--;

ptabella++;
}

if( --prescaler_50ms == 0 )
{
prescaler_50ms = VALUE_50MS;
ptabella = (unsigned char data*)&timer50ms;

for( i = 0; i < LENTIMER50MS; i++ )
{
if( (*ptabella) !=0 )
(*ptabella)--;

ptabella++;
}

if( --prescaler_1sec == 0 )
{
prescaler_1sec = VALUE_1SEC;
ptabella =(unsigned char data*)&timer1sec;
for( i = 0; i < LENTIMER1SEC; i++ )
{
if( (*ptabella) !=0 )
{
(*ptabella) --;
}
ptabella++;
}

}

}
}