I tend to have any adverse reaction that is clinically possible - my body really dislikes pharmaceutical drugs so don't take my word as the final word, it was more a word of caution - but cardiac events do appear to be a problem with memantine.
I switched to DXM starting last night by taking 5mL of Delsym at midnight. I then SKIPPED my Valium dose and took another 5mL at 6AM then fell asleep about 11AM. I woke up about 3PM, which meant I missed my afternoon dose of Valium as well and then about 4:30 I finally took 5mg of Valium and that's all I've had today. I took another 5mL of Delsym when I woke up and another 5mL at 7:30 or so and will take another 5mL at midnight. Normally I'd have taken 20mg of Valium in this time period so there is definitely something to dextromethorphan versus memantine. I should also note that I have experienced no respiratory suppression or heart abnormalities since I stopped taking the memantine, even with the extreme decrease in Valium.
Even more amazing is the fact that when all this went on with the heart racing out of control on the MDMA/Mescaline combo and the drinking that ensued, I ended up taking a full extra 20mg of Valium that day but I'm still able to come down so quickly off of it the day after the fact.
As for the tolerance prevention, just go back on the memantine after you come down from the MDMA. Memantine has quite a long half-life though so there's still a chance that a good bit of it could be in your system even a week later. Even at the low side of 60 hours, that's 2.5 days before your blood level of memantine is halved and another 2.5 days before its quartered, meaning that it takes 5 days to get down to 10mg plasma levels after taking 40mg. On the high end, the half life is 100 hours so you'd be looking at just over 4 days before your plasma levels would hit 20mg after dosing 40mg.
Either way, it seems that all of the NMDA antagonists have the ability to cause severe CNS issues including respiratory failure, heart attack, and stroke, so just be careful with them. Ketamine has been known to cause significant increases in pulse and blood pressure and everyone knows that recreational levels of dextromethorphan cause tachycardia and hypertension.
All that being said, the 120mg of dextromethorphan polisterex I have taken is having less of an impact on my pulse and blood pressure than 40mg of memantine did.