Missfire under load

Car was running great and slowly developed a miss under load . Its starts at about 3400 rpm,seems to be one cylinder, plugs look good ,valve lash looks good .Here's the kicker I'm running a early 90s vertex mag and a 650 holley.As I'm typing I think I'll check the fuel filter. But anyhow I want to rule out the mag any ideas? I'm going to put it back together and put a thermal light on each header tube to isolate the cylinder.

misfire under a load sounds like plugs or wires

I'd look at each wire ( pull out of cap one at a time) Also off each plug and I bet there is a bad crimp or corrosion on one.
Ron

I may have found the problem, the spring loaded contact to the rotor seems to be to loose and sloppy! I have a new cap and rotor coming thanks for the input! Also I checked everything that was mentioned.
Thanks Guys I'll let you know if that's it