We've had our UP! 3D Printer since April 2011. It printed great for quite a while but recently it has become increasingly difficult to use.  The problem is generally with the extruder not working. We typically check that the printer extrudes correctly by testing the extrusion function before printing. (this generally passes with flying colors). Once we start printing, the results are variable. Sometimes it fails to even start to print the raft, and sometimes it quits printing at some point during the model but it rarely prints all the way through without stopping. In order to fix this, I've done the following and we still have issues.
Replaced the ABS roll with a new one
Replaced the nozzle
Replaced the entire print head.
Does any one have any suggestions as to what might be causing this problem? It is quite frustrating.We have also very recently noticed that the Nozzle Height seems to be Changing between print jobs. After printing one model semi-successfully, the next one causes the nozzle to slam into the table. It stays messed up even after re-initializing. Is anyone else having similar issues? Do you have any suggestions on what might be happening?

Any help will be apprecited.

I didn't find the right solution from the Internet.

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Thank you.