Hi! start with the physical level:
Was/Is that Cat. 5E cable (so made for 100 Mbps / Fast Ethernet not for 1000 Mbps / Gigabit Ethernet) certified end-to-end with an instrument (RJ45 connectors, cable pairs connection scheme and cable lenght are then OK)?
If the Cat. 5E cable is just a long pre-fabricated patch cord "n" meters long you can presume it is certified at origin but doing a check wouldn't be a bad idea.
Then if you connect an host (a desktop or a notebook) on that cable directly on one of its ends (remote or local, no matter), so involving just one switch (8 ports or 24 ports), how the traffic flow behaves (did you test with iperf for verifying host-to-host connection speeds and with ping for verifying ping responses delay time or packets loss rate) against that switch and its connected hosts?
If you invert the test does something change? ...this to exclude an issue on a specific Switch if the cable has proved to be really OK.
If you're experiencing issues with just one uplink cable (which is not technically a Trunk, just an Uplink) between your two switches then aggregating (trunking) more physical ports to form a single logical Trunk is not the solution to your current issue (even if you find/have other intra-Switches free cables to use for that).
Edit: maybe this thread should be moved to "ProCurve / ProVision-Based" or "Web and Unmanaged" sub-category since the HP ProCurve 1800 Switch series (members: 8 ports J9029A - firmware PA.03.10 of 12/2013 - and 24 ports J9028A/B - latest firmware PB.03.10 of 12/2013) historically should belong to ProCurve and its switches are web manageable.