@acer 

This is research project of mine, I'd like to keep the code confidential. That being said, I also (perhaps wrongly) figured that if the code worked sometimes, the issue was not related to the code itself. If you would be so kind to look at my code, I can send it to you privately.

Thank you.

@Carl Love To 20 digits of precision it does indeed run very slowly. I'm going to let it finish one round of simulations so I have the data, but then I will certainly explore other methods.

@vv Can you explain why increasing the number of digits made everything work out just fine?

Also, not so sure that rkf45 is the best method, it was a suggestion from my supervisor. I'll give it another try, allowing dsolve to choose.

Thank you so much for your insight!