shoebox shuts off before filling tank

I have an older 4,500 psi shoebox I bought about 4 years ago,but I only use it about once or twice a month.I lubricate it every time I use it with Lithium grease and have it set to fill my 4,500 psi Guppy tank (smaller field one) from 4,500 to 4,800 psi. It is the chain drive with the hardened axle.Recently when I've been using it it shuts off at various psi fills before 4,500 psi and I had to turn it back on twice to get it to fill all the way. Before I dare touch it could it just be overheating or maybe the chain has streched? Any advice is appreciated because messing with high pressure compressors scares the heck out of me and my .25 Marauder and .25 HatsanBT65-SB I only fill to max of 3,100 psi but I just got a custom High powered .308 that can fill well over 4,700 psi.

there is a wealth of information in the shoebox section of this forum, but when my shoebox was shutting off prematurely. Tom , the owner of shoebox, said that my lower cylinder needed to be rebuilt. you can get a complete rebuild kit for underr 30 bucks from Tom,iirc