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We investigate the growth over time of 20 massive (> 3 keV) clusters in a hydrodynamical simulation of the LambdaCDM cosmology with radiative cooling. The clusters show a variety of formation histories: some accrete most of their mass in major mergers; others more gradually. During major mergers the long-term (temporally-smoothed) luminosity increases such that the cluster moves approximately along the Lx-Tx relation; between times it slowly decreases, tracking the drift of the Lx-Tx relation. We identify several different kinds of short-term luminosity and temperature fluctuations associated with major mergers including double-peaked mergers in which the global intracluster medium merges first (Lx and Tx increase together) and then the cluster cores merge (Lx increases and Tx decreases). At both luminosity peaks, clusters tend to appear spherical and relaxed, which may lead to biases in high-redshift, flux-limited samples. There is no simple relationship between scatter in the Lx-Tx relation and either recent or overall merger activity or cluster formation time. The scatter in the Lx-M and Tx-M relations is reduced if properties are measured within R_500 rather than R_vir., Comment: Accepted for publication in MNRAS, major revision including greater statistical analysis. 17 pages, 17 figures