I notice this same effect. Sometimes I dose and then all I remember is a few foggy images, like a dream, and then I wake up on the floor somewhere wondering what damn day of the week it is. I'm not sure if caffeine or a mild stimulant would be good. I read something about alpha-2 agonists increasing the Olney's lesion effect. I'm no doctor but I'm pretty sure just about any stimulant would increase either adrenaline and/or alpha-2 receptor activation. Then again, if you go to the erowid page on DXM, there's another article saying Olney's lesions don't occur. This whole Olney's lesion hypothesis sounds like another MDxx: no one really knows what dosage, combos, or environments might permanently damage the brain.
I'll admit I ended up with anxiety, loss of short-term memory and focus, and emotional instability when I started using drugs. But slowly with my years of experimentation I began to filter out which ones were the real culprits, and how often and how much I could get away with. Ironically, it turned out most of my problems went away when I took a 2 year break from pot, supposedly the most harmless.
Anyway, DXM is not very consistant at creating 4th-plateau experiences, and this is probably due to the whole list of other receptors it effects, the way it disrupts metabolism, and just by looking at the way it mixes with others I can tell this isn't just a dissociative.