PCR improvement with Phusion Hi Fi taq polymerase

Hello, I'm doing a PCR with Phusion taq polymnerase with a cDNA (from RT) of 5 kb. I get a lot of non specific bands in addition of the desired one. I tried to do a temperature gradient but it didn't improve. It got better using GC buffer, but only a little bit. Is there any enhancer that works with this enzyme?