Have you seen a specialist in digestion issues?
At least you know Lucozade works for you so you have something. I prefer Lucozade too. With the dextrose tablets, perhaps letting them sit in your mouth crunched up will help as we can absorb a little glucose through the mouth.
I don't know what else to suggest other than seeing a gastro specialist and making sure your doctor has screened you for common conditions.
I hope your DSN visit gives you some answers.