Many ISP will have a pool of recursive servers for their clients : in part for load balancing, in part redundancy in case one or more fails. It's not unknown for there to be configuration differences between the various servers which might make some more "forgiving" than others which would explain why it works for while, occasionally.

Sometimes, ISP will even use a diversity of hardware and/or software also to protect against bugs/defects/vulnerabilities in the respective implementations and that too would potentially explain the observed behaviour.

This is nothing but speculation of course ... the real corrective action is, as you have done, get the issues addressed on the authoritative server (which ideally should be servers!)

In reply to a post by bowdon:

I managed to get to both sites.

I'm on BT and using Googles DNS servers.

Same here, I can also access them both also using googles DNS, however googles DNS Servers are slow now and then and then sometimes times out.

Paul

Looks like a routing problem to me

Tracing route to arktelecom.com [64.118.199.100]
over a maximum of 30 hops:

1 <1 ms <1 ms <1 ms VIGORG6CRV [192.168.1.1]
2 6 ms 6 ms 6 ms losubs.subs.dsl4.wh-man.zen.net.uk [62.3.83.6]
3 6 ms 6 ms 6 ms ae1-120.cr2.wh-man.zen.net.uk [62.3.86.9]
4 7 ms 6 ms 6 ms ae2-117.cr1-man1.ip4.gtt.net [77.67.66.101]
5 86 ms 86 ms 86 ms xe-0-2-4.cr8-nyc3.ip4.gtt.net [89.149.129.13]
6 86 ms 86 ms 86 ms 206.111.13.84.ptr.us.xo.net [206.111.13.84]
7 114 ms 89 ms 89 ms 216.156.16.132.ptr.us.xo.net [216.156.16.132]
8 91 ms 92 ms 90 ms te-4-2-0.rar3.chicago-il.us.xo.net [207.88.12.13
7]
9 89 ms 89 ms 89 ms 216.156.16.125.ptr.us.xo.net [216.156.16.125]
10 * * 165.117.253.98 reports: Destination net unreachable.

Trace complete.

In addition to ns1.sysnet.net returning poorly configured records, ns2.sysnet.net, which is listed at the parent nameservers, isn't even responding. Whoever configured those DNS records made a complete mess of it.

Edited by Oliver341 (Sun 30-Apr-17 12:06:16)

I cant access those sites with my BT connection but my other line works fine! Hope you get it sorted.

BT ISP using OpenDNS. No problems, comes up quickly.

Three weeks on, the problem has probably been solved .

Actually, No, it's same as ever : some people will have access, others will not, Some may get variable access depends on exactly how your DNS servers have been configured. Internet service provider Russian roulette..

dig www.greece.org @ns1.sysnet.net

; <<>> DiG 9.10.3-P4-Debian <<>> www.greece.org @ns1.sysnet.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: FORMERR, id: 4579
;; flags: qr rd ra ad; QUERY: 0, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; WARNING: EDNS query returned status FORMERR - retry with '+noedns'

;; Query time: 93 msec
;; SERVER: 64.118.200.2#53(64.118.200.2)
;; WHEN: Sat May 20 19:30:46 BST 2017
;; MSG SIZE rcvd: 12

The OP's problem! As in the Subject. And his reply to you.

In reply to a post by kritifile:

... I don't know if this will be resolved, and I am wondering if some ISPs are more forgiving than others, but thanks for searching that out and I'm sending it over the pond as I have no access to the DNS servers there.

Edited by RobertoS (Sat 20-May-17 20:28:12)

It's not been solved in spite of many emails.
The strange thing is, every few weeks the domain resolves on BT for anything between a few minutes and a few hours, then I lose it again.
No problems using the Vodafone mobile network or using an anonymous web proxy but not everything can be done on the web.
I've tried using Google DNS with both the HH5 and a TP-Link that allows me to change DNS on the router as well as setting BT DNS manually, that gives me access to the server, but I have to change it back to default because nothing but greece.org and arktelecom.com resolve if I set DNS manually. With both Google and manually set BT DNS nothing else resolves!