Hi,
Can you set the 'sys mtu 1500' on the 3550 switch. 3550 switches support different MTU settings and I don't know what your's is set to. Most router interfaces' MTU default to 1500-bytes. Making the MTU setting consistent on the switch with other layer 3 devices in the network could avoid problems with fragmentation at various sizes.
Let us know if it helped.
HTH,
Sundar