I don't know which Uni filter you have, but is there a step-down from the main opening? I was using a pod filter, and it hesitated badly in 5th. As an experiment, I rode a short time without a filter, and it was fine. Then I noticed the pod I was using stepped down from the collar that goes over the carb mouth, causing a restriction. I got a different pod without the stepdown, and it's fine now. BTW I'm not riding in dirt, so feel okay using a pod, instead of oiled foam, though I know some may disagree :o)

I'm running the stock airbox with UNI foam wrapped on the stock cage. No pod.

That should work fine. If the mixture seems overly rich, try running the bike with fuel tap turned off, and see if it momentarily runs well, before cutting out.