I installed OC on a shared server and have problems with syncing and also viewing the llaendar/contacts online.
The host has said that the reason I am having problems is that the system is too resource intensive for a shared server and is constantly running out of resource, therefore creating the errors.
Why is OC so resource hungry - is there anything I can do to make it work properly.
I never used to have problems when I first used OC a couple of years ago - so has the system changed that much?
It is hard to say, we don't know how much resources you have, we don't know how much you use. In theory, you can run ownCloud on a RaspberryPi which is not very powerful and the performance is not the best. You can do a lot by tuning your server, using nginx instead of apache, check your database settings (the size of caches should be chosen carefully) and how much memory you dedicate to each php process (and how many you run). It's especially tricky if you have little resources. And there is certainly a point, where all tuning doesn't help and you need a more powerful server.
Some shared servers only guarantee a certain amount of resources and provide "up to" more depending on other customers. So it's perhaps not only you and ownCloud, your provider has put more customers on the machine or there are more power users among them.