I own a 1999 pantera with twin 502 efi's. Late start to season and had the water pumps, distributor, cap, and plugs changed. Ran right around 5000 rpm's last three years with gps reading between 76-79 mph. This year, port motor max rpm's right around 4200, starboard max around 4400 and I can only reach 64 mph. Engines don't seem to be running rough. Any ideas.

Thanks, Joe

Check your timing and the fuel filter.
pull the sparkplugs and look at them , u want a nice light brown color on the tip.
Also check the plugwires,,if 1 cyl,fails u really cant tell.

You didnt mention having the fuel filters and water seperators replaced. If they were not, I would start there.