Problems With Virtual Instruments, No sound on certain software instruments Options

[cgt18]

My first post here but have been enjoying the daily posts for some time now...thank you for that!!

I am experiencing a major problem with some of my Software instruments.

I am running Mac OS X 10.4.5
Logic Pro 6.4.1
17" Powerbook 1GHz, 1 Gig of RAM,
Motu 828 Mk2
TC Powercore Firewire
Lacie External HD (Firewire 800) 160Gig...

I can use the EVB3, the ES1, ESM, ESP, ES E, ES2, EXSP24 & EVOC perfectly normally.

BUT NOT The EVD6, EVP88, EVP73, EXS24, which only sound as a distant, dull drone almost devoid of all pitch and virtually none of
the original tone of the instruments.

The synth on the Powercore is ok.

However the instruments sound fine if they are playing back pre-existing midi or if I play something in
parts (as long as i deactivate the record button on the instrument....)

I was slightly foxed at first because when this problem first appeared a month or more back I
mistakenly thought that my old Roland piano (RD 1000) was sending out screwed up MIDI of some sort.
I am certain that is a red herring because I can replicate the issue when I use my Oxygen keyboard or try to play back some existing MIDI tracks.

I tried a re-install from my Logic Pro cd. No different.
I downloaded the latest MOTU stuff ..
also repaired permissions....all to no avail.

Desperate for some guidance on this one.
Thank you!