It may be worthwhile checking your PSU outputs ...the USB sockets need to provide a nominal 5VDC - and the PSU output shopuld be at all times within +/- 5% of the nominal.
A weak PSU may be close to the edge in tolerances...I don't know how that might affect such a RB ..but has to be worth checking in view of the "Weak light"
You can check easily using Motherboard Monitor (Free- Google it) which will give readiouts of all voltages (12/5/3.3VDC,) CPU temp, system temp,fan speeds etc for most motherboards.