It could be a stackproblems, but I suggest that you start your
interruptroutines with 'CLR ETx' in ext. int-routines, and
'CLR EXx' in timer-int-routines so you make sure that you
slake out the int-routines you're not using.
But make sure that you also ends the routines to 'SETB' specific
interrupt.
It can results that you get some priority problems, but i'ts
only an idea worth to test.!