Another interferer can be portable electrical devices that aren`t similarly shielded, home phones are a big culprit by nature they`re delibrately emitting signals - but these can often be bypassed by selecting different channels to operate on.
2.4Ghz phones will interfere, but other types won't. Leaky microwaves will though. Interference won't usually cause dropouts though, that's most commonly due to the router overheating.
