Because you are mixing 'C' and assembler it is possible that your irq handler is overwriting a register used by the compiler. I cannot comment for other compilers but in Keil you can use 'C' functions to handle interrupts, eg:

void tim2_irq(void) interrupt 6 using 1
{
TF2=0;
counter++;
}

Which will associate the timer 2 interrupt vector with the above function. Specifying 'using 1' will make the compiler reserve register bank 1 for the irq handler which may reduce the number of registers that need to be pushed and popped. Irrespective of whether you specify the 'using' directive the compiler will push/pop whatever registers are necessary and end the function with an iret.