Original charger and cable? Slow charging likely means the phone thinks it's connected to USB rather than to an AC source. Some car chargers also can't supply a high enough current to overcome the load of say, running navigation software, on the battery. Could be charger, cable or phone at fault - try to eliminate the first two by swapping them out?
I'm going to swap out to double check - but the issue of it decreasing when used and plugged in? This happens at any percentage ... if I plug it in at 20% and use it, it'll drop
Dolby- are you running a custom ROM and kernel by any chance? I had a similar issue, where it charged exceptionally slow, so slowly in fact, that the phone often died whilst charging.