I suggest to have your cables checked by a company that has the right equipment (Fluke?). They should be able to give a detailed report on the capacity and quality of each cable and of the switches. CAT5 by itself should be able to do it, it only says something about cable quality, not about wiring quality.
Once I saw an office go from 100 Mbit to 10 Mbit after they replaced their 100Mbit hub with a gigabit switch. Huh? From the report, it appeared that the initial CAT5 cabling was non-EIA strung (all pairs were crimped as parallel, not crossed as required by EIA-B) and the cables were snugly tied to metal heating tubes using tie-wraps, making crosstalk and standing wave nodes cripple communication. Rewiring, recrimping and rearranging cables resulted in an A-grade report and a network with over 80MB/s transfer rates.
Of course, you could do your own preliminary tests by having two up-to-date computers attached through the longest path of the network, both in terms of physical distance and in node hops, and time the coping of a huge file.