Which I suggest strongly that you rectify at your earliest opportunity. The Mac is designed for interleaved (paired) DIMMS, either EDO (presently cheaper) or FPM, but definitely not mixed EDO and FPM, installed (in pairs if you are using interleaving) from highest slot numbers downwards towards the lowest.
What you describe is alone capable of producing any number of erratic and unexpected responses as the system tries to allocate memory over a groundplan that leaps from precipice to plain.
4 x 128MB EDO 168-pin 60ns DIMMs are readily obtainable, and may signal the dawn of a new era for your Mac.
de