Have you tried techniques for dealing with forceful letdown, rather than oversupply? They are related but not the same thing, and the 'symptoms" you describe could be an issue caused by forceful letdown-and thus, just dealing with the forceful letdowm could alleviate this-
I have suspected milk supply issues so about week ago I started nursing the newborn and 3.5 yr old on separate sides. This has helped with oversupply, but he still spits up a lot, and gags on my milk, even if I'm laying down. I can also tell this causes reflux (cries a lot) and I haven't noticed improvements from my diet, but will be trying eliminating my daily can of coke which I suspect he may be sensitive to caffeine. He has plenty of wet diapers and is gaining well.
How do I adjust my milk supply if I'm already nursing on one side? Does anyone have experience tandeming on separate sides? I'm sorry my question is multi-faceted and could be posted in multiple forums.