I am recieving an error 22 at the printer after the 20th page of a big print job all the time.
Small jobs print flawlessly.
As an obvious first step, see if a shorter serial connection works, and make sure the connection is clear of any interference generators such as power cables. You will want to contact the customer support center for assistance. Expect to be asked the firmware version of the DECserver, as well as information on the HP LaserJet 4si, as well as for specific symptoms.