Lear Autopilot

:-newbieI apologise in advance for the fact that this may well have been discussed (many?) times before...But...as a newcomer to the default Learjet 45 (it's only taken me about 2 years to get round to the bizjet thing), I'm a bit worried by the fact that the autopilot seems to have some serious problems in maintaining height when, for instance, the aircraft is changing heading...It appears to have an annoying habit of constantly 'nodding' up and down by up to 4- to 500 feet - which attracts the attention of ATC every d*mn time telling me to either ascend or descend blah blah blah...Like I say, I'm sure this has been dealt with before, but I honestly can't find any mention of it...Any help in sorting this out would be appreciated. I do assume, of course, that this isn't 'natural' behaviour by the Lear 45...otherwise the passengers would end up feeling rather ill... :-erks Pls help!!!!!