You have encountered that rare situation where the small differences in the
internal timing of the two RAM modules makes them incompatible. I doubt if
there is anything that can be done to makes them work reliably in your
system. Are they both made by the same manufacturer? (I doubt it.)

It is not a cost effective solution, and no guarantees, but probably two
identical new, or used, sticks would work perfectly. Identical is the key.
Otherwise just use one stick, which will probably give satisfactory
performance. I'm running 11.04 on my old 2GHz laptop with one GB of RAM and
it seems snappy enough.