I noticed with my time capsule 2TB that in certain spots in my apartment the signal strength dropped significantly.
Chances are, those spots are located where the wireless signal must pass through several walls to travel from the Time Capsule to your iMac, correct?
If yes, how many walls must the wireles signal pass through?
What is the construction type of those walls?
Two walls will absorb, on average, 40-50% of the wireless signal strength.
If you think the problems are elsewhere, then you are likely picking up wireless interference from another new WiFi network near you, cordless phones, other electronic devices, etc.
See this Apple support document for more details and suggestions: