... the 400w model has only 3A but the next model in the series (500w) has nearly 3 times more - 8A
The numbers do not make any sense unless you are talking about AC input power which really does not matter too much.
The key figures are 12 volt capacity. A bit of googling indicates that the 400 watt PSU has two 12 volt rails, each with a maximum of 18 amps. However, total capacity is 300 watts or 25 amps.
You haven't told us the rest of your system spec, particularly the video card.
Novatech's are not particularly high quality units. Yours could either be defective or overloaded by a video card that needs more power than the PSU can provide.