Those Q-Tech PSUs are good quality, and are also well made, and most of all cheap (and very popular). They are also quite adequate for business PCs, budget family PC's, and any PC that would have been manufactured about 2 years ago.
However, these cheap PSUs are not suitable for enthusiasts (power users, and those that seek performance, or even many of todays PCs), and it is very misleading that they are advertised with such high wattage rating, as this implies that they are up to any task. This misrepresentation is common with cheap PSUs, and like any product, quality costs money.
Stick with top brands are you won't go far wrong, though ideally you still need to know what to look for when buying (even *some* quality 'Antec' brand 400w PSUs only have 18A@+12v, and Antec is a highly regarded brand), and match your choice to the power requirements of your system.
Note that not all 300w PSUs are created equal, and some have been known to output as low as 120w, due to inefficient power conversion and phase shifting of output voltage/current (please excuse the jargon). Better PSU's are often labelled 'PFC', which stands for Power Factor Correction, a method to correct the voltage/current phase shifts and so make more of the power available to the load (your PC).
Good to hear that you have finally solved your problem.