Running an 0-32O with 2 mags-frequently get a 200 RPM (or sometimes greater) drop with  roughness on the left Mag when doing run ups.

Until recently I had the MATCO brakes installed I was unable to run up much past 2000 RPM without creeping forward and thus the problem was hard to clear. On applying full power for take off-the engine runs a little rough for a couple of seconds then runs smooth.

Obviously the cause is fouling-anyone care to add comments or similar experiences?

I always taxi with mixture leaned out to the max but mechanics can be more careless when picking up aircraft for maintenance.

What are your thoughts?