It could be postprandial hypotension, a drop in blood pressure that occurs after eating which can result in presyncope (dizziness, lightheadedness, weakness, almost fainting) or syncope (fainting) in some patients. A holter monitor does not measure blood pressure, so it would not rule out postprandial hypotension as a cause for syncope. You can ask for ambulatory (24 hour) blood pressure monitoring as a test for this, or you can get your own cuff at home and check up on your blood pressures before, during, and after meals and show your doctor if you do indeed have a drop in blood pressure that correlates with your dizziness and fainting spells after eating. If you are having syncope at times other than just after eating, tilt table testing may also be warranted.
You may find helpful information in the "Grubb Overview on Syncope (Fainting) from Mechanisms to Treatments" found in the Dysautonomia Community's Health pages here:
http://www.medhelp.org/health_pages/Neurological-Disorders/Further-Reading-on-Dysautonomia/show/696?cid=196
http://www.medhelp.org/health_pages/Neurological-Disorders/Autonomic-Dysfunction--Dysautonomia-Index-Page/show/857?cid=196
We'd be happy to answer any questions you have about syncope over at the dysautonomia community:
http://www.medhelp.org/forums/Dysautonomia-Autonomic-Dysfunction/show/266