Hmm a lot of stuff can cause inconsistent speed when using wireless connection. Is your router firmware up to date? Is your QoS enable? Do you have another brand router to test and compare with? I know i had a similar type of issue before. I solved mine by switching my wireless channel. If your neigbour around you just so happen are using the same channel as you that might cause some issue thought not very big. I solved mine by downloading this to scan for congested channel used around my neighbour.
inSSIDer – Discover The Wi-Fi Around You | MetaGeek
I would suggest getting another type router to test it out with. Since the issue occur right after you said you replaced your WRT54GL.