I understand your problem very well. I have four Netmondo sites and their response has been sporadic recently. However, that is OK for me because all my sites are for small classes and teacher associations. I think Netmondo is perfect for that.
For production servers with large classes, I have never recommended Netmondo, and I think no one has on these forums. For $29/year, do not expect so much. For large classes you need your own server or a hosting service with a lot of capacity and service. That kind of service costs money. Moodle.com offers the best service and at $2400/year it is a bargain.
Now you said you gave an exam for 1800 students. Absolutely, to do that you need your own servers. Netmondo did not suspend your service, you totally overloaded it. I did the same kind of exam on my server and the whole system shut down.
If you gave say, a 50 question exam to even 80 students simultaneously, you will need a minumun of one very high performance dedicated server machine from my experience. I could use a 1.2ghz Mac as a server for up to 80 students simultaneously. Now I have a 3.2ghz Linux with 2gb memory and can do 100-150 students simultaneously. Quizzes are a very heavy load, and simultaneous testing is extremely heavy. I recommend you purchase 1-2 servers or consider doing the test with fewer students at the same time. Please feel free to ask more questions about this and good luck
P.S. Netmondo recently purchased two other hosting companies and have not been able to cope with handling the service load despite hiring 4 more service technicians. I agree with Martin, that moodlers should go to Moodle Partners
(or don't complain when things go bad with inexpensive providers