I have no knowledge of the RasPi hardware specifically, but a slow drift in voltage usually has something to do with temperature- something heating up, or cooling down. For example, a voltage reference could heat up and drift, a bad connection could become better or worse due to temperature change shifting contacts ever-so-slightly.
A polyfuse (resettable fuse) which has been "tripped" due to overcurrent recovers gradually over time, as it cools off and allows more current to pass, see for example http://en.wikipedia.org/wiki/R.....table_fuse
Hopefully the boards do not have out-of-spec fuses installed, but I'd think that is one possible explanation of the symptoms reported here.
are you saying i should let the device cool down for a few hours as I might have tripped it and not give it time to self heal? (its been on since i got home 6hrs ago!)
Its late anyway so going to bed, will report back tomorrow!