When I'm driving the lights and radio dim or go.

The generator has to be tested while the problem is occurring and it needs a professional load-test. If one of the six internal diodes fails the unit will only develop exactly one third of its rated output current, but a simple and quick voltage test will show it's fine.

These generators have an extremely high failure rate due to their design. They develop very high voltage spikes that destroy the diodes, the voltage regulator, and can interfere with computer sensor signals. To reduce the number of repeat failures replace the battery at the same time unless it is less than about two years old. As they age they lose their ability to dampen and absorb those spikes.