I note it says it has an access time of 150 ns. If you look at the spec's for the 805x-series, particularly your specific MCU, you must be sure that can get "around the horn" within that 150 ns window. The 805x does TWO code-space accesses on each external code memory cycle, but uses only six clock ticks for each. It uses the first ~3 to generate the memory address, and then uses the remaining three to perform the read. If you're still bent on "pushing" your MCU to ~18 MHz, you may find that the 150 ns window isn't enough performance. After all, you have only 163 ns for the whole access time, and must allow something for propagation delays.

Run the numbers! Watch the oscilloscope! You may find you have a code-memory-READ-cycle problem.

RE