I had the same problem on my BME 102 twin. I asked around the IMAC list and this is what the problem was:

BME's like the low needle to be LEAN. Turn in your low needle as much as possible (until it won't transition OK, or just plane quits or won't start). Set the high needle to rich for breaking in.

As far as I know, the poblem does not show up on the ground, or on the bench, only in the air. Mine had the RPMs going up and down, up and down when at about 1/2 throttle. This is where the high needle kicks in.

After leaning out my low needle, my BME 102 runs like a dream.

Regards,
Craig.