pf: incorrect cksum

Hello.

I have an office router sun4v sparc SUNW,Sun-Fire-T2000 with SRU 11.3.5.6.0, and sometimes, users can't download files. After some invetsigation, I saw next: on start everything is ok:

10:58:26.112941 IP (tos 0x0, ttl 60, id 5130, offset 0, flags [DF], proto TCP (6), length 1500)
    213.133.190.40.80 > 10.5.7.39.53261: Flags [.], cksum 0xc303 (correct), seq 32175205:32176665, ack 0, win 4106, length 1460: HTTP
10:58:26.112998 IP (tos 0x0, ttl 60, id 5131, offset 0, flags [DF], proto TCP (6), length 1320)
    213.133.190.40.80 > 10.5.7.39.53261: Flags [.], cksum 0xa289 (correct), seq 32176665:32177945, ack 0, win 4106, length 1280: HTTP
10:58:26.113217 IP (tos 0x0, ttl 60, id 5136, offset 0, flags [DF], proto TCP (6), length 1500)
    213.133.190.40.80 > 10.5.7.39.53261: Flags [.], cksum 0xb764 (correct), seq 32177945:32179405, ack 0, win 4106, length 1460: HTTP

but after some times, I see incorrect cksum:

10:48:07.706995 IP (tos 0x0, ttl 60, id 58798, offset 0, flags [DF], proto TCP (6), length 1500)
    213.133.190.40.80 > 10.5.7.39.53004: Flags [.], cksum 0x051a (incorrect -> 0x0519), seq 1400912173:1400913633, ack 515535425, win 4106, length 1460: HTTP
10:48:15.589114 IP (tos 0x0, ttl 60, id 7160, offset 0, flags [DF], proto TCP (6), length 1500)
    213.133.190.40.80 > 10.5.7.39.53004: Flags [.], cksum 0x051a (incorrect -> 0x0519), seq 0:1460, ack 1, win 4106, length 1460: HTTP
10:48:31.153359 IP (tos 0x0, ttl 60, id 38260, offset 0, flags [DF], proto TCP (6), length 1500)
    213.133.190.40.80 > 10.5.7.39.53004: Flags [.], cksum 0x051a (incorrect -> 0x0519), seq 0:1460, ack