04 300 sundancer port engine shuts off

we will be under full throttle and port engine shuts off. It will come back on a few times for a short period then not at all. Next the same thing happens it will stay running at 2500 rpms, but if you take it to above for it shuts down. Now its having problems with staying on during engine idle. Looking for ideas. my mechanics have not had any luck.

I've had both an ignition switch and ignition relay cause this same problem.