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As noted in other posts, getting 2nd stack cable in operation will double bandwidth between stack members. However, 3750 series are noted for insufficient port buffers to handle (well) transient bursts, so I would first suspect issues there rather than on the stack (but it's still worthwhile to get the 2nd stack port in operation.)
Look for packet drops on ports. If there's much, check whether QoS is enabled. If so, determine whether you need QoS. If not, disable QoS. If needed, you may need to tune buffer usage parameters.
Also on the 3750X series, the "uplink" ports have more memory for the number of ports than the copper ports do. So if you have high drop rate ports (SAN devices?), if not already using them, move those ports to available "uplink" ports (if copper, you'll need copper SFPs).
Other alternative for very high usage gig ports, if possible, consider Etherchannel and/or 10g.
Leo's suggestion to upgrade IOS, besides addressing bugs, may (cough - NDA - cough) offer some other improvements.
32 Gb/s with a full or complete StackWise ring
