This problem can happen if the TCP sessions are being dropped.
When a host (usually a PC) initiates a TCP session with a server, it negotiates the IP segment size through the Maximum Segment Size (MSS) option field in the TCP SYN packet. The value of the MSS field is determined by the Maximum Transmission Unit (MTU) configuration on the host. The default MSS value for a PC is 1500 bytes.
As a workaround, adjust the MSS value of the TCP SYN packets going through the router. In order to do this, issue the ip tcp adjust-mss command in interface configuration mode.