Sending MMC to Pro tools causes audio throughput to stop

I'm having horrendous issues trying to sync Pro tools as a slave to a master clock coming from an MPC4000 via an RME UFX (MTC + MMC) - i think i've isolated it to being something to do with MMC (although it could be somewhat time code related too). Basically I can see audio on the input pro tools meters but as soon as pro tools is put into online mode the level disappears... likewise when no audio is playing and i start the MPC you see a single spike on the pro tools meters and then nothing (although interestingly sometimes there appears to be a second spike several seconds later). I should point out the RME shows input is still getting that far but inputs within pro tools and therefore the outputs on the RME go silent
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This is driving me crazy - has anyone got any ideas? (No I don't want to run pro tools as the master)

Sending MMC to Pro tools causes audio throughput to stop