Start the engine and turn on your lights. Get someone to stand in front of the car and rev the engine to a fast idle (1500~1800rpm). If the lights get brighter as the revs increase then your alternator's regulator is the problem.
It should allow more current to the battery at low revs and under electrical load.. but reduce the current at high rpm, to avoid over-charging the battery and cooking it. Your symptoms sound like the alternator is not reacting to an increased electrical load.
If you had a new alternator and battery a few years ago, it could be a coincidence or it could not. If your alternator is unbranded or has castings/labels of Marelli, Bosch and Lucas etc. on different parts, then it's been rebuilt, even if it looks new. The regulator could be a quality European replacement or it could be made in a shed in China. You want to see a label on it, ideally with a name of a manufacturer that you've heard of.
If your alternator was definitely a brand new, all-made-by-Denso jobbie, then it's likely to be okay in which case I would look at the earth strap.. not just the connectors but the inner cable itself. If you can find a new one for not much money, I would even just change it as a precaution. Often the cable strands corrode inside the plastic jacket, so you can't see that there's anything wrong with it. If you bend yours about and hear crackling, then that's not a good sign.
Ralf S.
