several possibilities:

are register banks getting switched unexpectedly?

if you have multiple DPTRs, are they getting switched unexpectedly?

is a dodgy interrupt overwriting your data?

is a dodgy pointer elsewhere overwriting your data?

Overlay problems can cause this sort of thing - especially if you're using function pointers. Search the Keil knowledge base & discussion forum.

are you sure you're correctly setting & dereferencing the pointer - remember that 'C' assumes that everything is an 'int' unless advised otherwise!