72 CL350 Bogging down in 5th gear

Check voltage at coils with engine running should be very close to battery voltage. Check voltage at higher rpm as that is when demand is high. Clean all connections in headlight bucket, kill switch is major culprit for resistance, connection at key switch. I have had partially broken wire from bucket to coils cause me headaches at higher speeds under load. Showed up when I direct wired the coils from the battery as a test. Easy to replace but a head scratcher.
Replace coils if original or you don't know age. Direct replacements are available but have the same color code but simply to figure out.
Look also at the condensor and points.

Perfect thanks I'll check it all out over the weekend hopefully one or more will sort it.

I would also remove the spark plug caps and ohm them, spec is 5000 ohms. Trim off some plug wire insulation, I like to bend the wires over like a waterfall then screw the caps in tight. Bending the wires over keeps the cap screw going into the center. Also check that you aren't running resistor plugs with the resistor caps. Many of us have switched to non resistor caps and resistor plugs as the old NGK non resistor plugs are nla.