Your domain is pointing to the correct nameservers which are pointing to the correct A record. You can verify this for yourself:

http://www.intodns.com/xoops.org

If it is a DNS problem, it lies somewhere at the resolution end. I assure you, your DNS settings are correct. If this truly were a DNS issue at the nameserver or registrar level, then none of your users would be able to view the site.

If there are any server side issues, this would have to be a malicious redirect or iframe insertion, something which I can find no evidence of on the server.

I am happy to continue working with you on this issue, but until we can reproduce this somehow, we just won't be able to get to the bottom of this. Have your users contacted their local ISPs regarding this issue? I would be interested to see what their local ISPs have to say about this issue.

I assure you, your DNS settings are correct.

Warn Same Glue Looks like the A records (the GLUE) got from the parent zone check are different than the ones got from your nameservers. You have to make sure your parent server has the same NS records for your zone as you do.I detected some problems as follows:
For ns1.xoopsproject.org the parent reported: ['66.7.217.147'] and your nameservers reported: ['66.7.217.148']
For ns2.xoopsproject.org the parent reported: ['66.7.217.147'] and your nameservers reported: ['66.7.217.149']

Different subnets WARNING: Not all of your nameservers are in different subnets

SOA EXPIRE Your SOA EXPIRE number is: 3600000. That is NOT OK

; <<>> DiG 9.3.4-P1 <<>> xoops.org
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47685
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;xoops.org. IN A

;; ANSWER SECTION:
xoops.org. 298 IN A 66.7.217.147

;; AUTHORITY SECTION:
xoops.org. 85024 IN NS ns1.xoopsproject.org.
xoops.org. 85024 IN NS ns2.xoopsproject.org.

;; Query time: 1 msec
;; SERVER: 83.149.105.188#53(83.149.105.188)
;; WHEN: Wed May 27 06:38:45 2009
;; MSG SIZE rcvd: 92

; <<>> DiG 9.3.4-P1 <<>> xoops.org
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28820
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;xoops.org. IN A

;; ANSWER SECTION:
xoops.org. 5033 IN A 66.7.212.213

;; AUTHORITY SECTION:
xoops.org. 5028 IN NS ns1.xoopsproject.org.
xoops.org. 5028 IN NS ns2.xoopsproject.org.

;; ADDITIONAL SECTION:
ns1.xoopsproject.org. 539 IN A 66.7.212.213
ns2.xoopsproject.org. 539 IN A 220.194.46.144

;; Query time: 1 msec
;; SERVER: 85.12.15.250#53(85.12.15.250)
;; WHEN: Tue May 26 20:51:58 2009
;; MSG SIZE rcvd: 124

; <<>> DiG 9.3.4-P1 <<>> paulohenriqueamorim.com.br
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31727
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;; QUESTION SECTION:
;paulohenriqueamorim.com.br. IN A

;; ANSWER SECTION:
paulohenriqueamorim.com.br. 3598 IN A 66.7.212.213

;; AUTHORITY SECTION:
paulohenriqueamorim.com.br. 86398 IN NS ns3.paulohenriqueamorim.com.br.
paulohenriqueamorim.com.br. 86398 IN NS ns1.paulohenriqueamorim.com.br.

;; Query time: 1 msec
;; SERVER: 83.149.105.188#53(83.149.105.188)
;; WHEN: Tue May 26 21:00:34 2009
;; MSG SIZE rcvd: 96

If there are any server side issues, this would have to be a malicious redirect or iframe insertion, something which I can find no evidence of on the server.