Mix down problem with session drummer 3 in Sonar X1

Hi,
 
Has anyone experienced this issue?  When mixing down audio with a Session Drummer midi file with step sequencer, the resulting track misses beats (often regularly like every fourth). This does not occur when I play the track 'live'.
 
The track is not too overloaded with audio and plug ins and theres nothing else running on the PC so its not
ram related and Ivwe got 4GB on a reasonable Dell box) but I think its something to do with the amount of info that SD is spitting out - like 16th notes s if I reduce them it seems to redcue the drop outs.  But this is not often a musical solution - and I notcie that drum rolls or anything on the last 1/16 are missed too.
 
Thnks
 
RF