yeah, it's the chlorpheniramine maleate that causes problems. it's an antihistamine, and in general antihistamines seem not to mix well with DXM. brompheniramine maleate is also a possibility, and other antihistamines may be present in other preparations (e.g. dimenhydrinate).
dextromethorphan HBr should be the only active ingredient. the product will be described as "dextromethorphan hydrobromide syrup USP".
the inert ingredients matter too. many syrups are "alcohol free" or "sucrose free". this sounds good, but for some reason, ones that are sucrose free make, er, some people very sick. its probably the thickeners rather than the sweeteners, but they're definitely not good in large amounts.