It sounds like you have no issues with your connection as-such, but just variable speed.
Unfortunately this is not, of itself, a fault - it is potentially just a symptom of varying amounts of use on the local network, manifesting itself as variable speeds.
Having said that, some of the speeds you've mentioned are below what I'd expect in most circumstances and the network status checker than @Northerner has suggested checking, would be an excellent place to start in this.
