Controller dead?

Hi. New guy to the forum. I have a Velocci electric bike. A beautiful piece made in the USA and I believe sold through NYC e-bikes. It is a 48volt with a 1500w motor. It stopped working. replaced the throttle switch with several different types and now it will run but cuts out and quits at anything above half throttle setting. It seems like the controller might be faulty. Factory replacements unavailable. Has anyone replaced these with another controller or know how to test/repair the original? Thanks Tom