2003 xtype.. why is my car jerking?

when first starting out she seems fine then starts to hesitate and speed drops
down to like 3 mi per hour with jerking like motions .. taking foot off gas and
reapplying gas she will sometimes just shoot out then die back down to 3 mi
and jerking motion foot to the floor ..and stay there for a bit then doing the
same when gas is reapplied shooting out like 3mi to 10 in like nothing flat
then dieing back and jerking again. She can sit for a bit then you go to drive
she seems fine nothing happens ... within a short time a few blocks or so
same problem. Nothing shows up on the diagnostics. even when this is
occurring. I crawled to my mechanics but it is so intermittent can't seem to
track down the problem