In short....Sticking Transfer roller solenoid..
Lets start with HERE
is the service manual, you will need it to understand me, no fear it is free today;)
is your PARTS manual
What the machine is trying to tell you is written on page 61 of service manual aka transfer roller aint lifting
The problem is SL5 its a soleniod (page 30 of your parts manual)
Yours works but sticks, what goes on as these age is there is or was a little pad that stops the metal from clicking when the arm of the solenoid hits home. a solenoid is just a spring loaded arm that is moved by a electro magnet, when the electro magnet it "turned on" the arm moves to the magnet and allows the transfer roller to turn, what yours is probally doing is when it is turned on there is a delay cause it is sticking so the whole timing goes out of whack. The pad on the solenoid goes all gunky and holds back the arm from getting attracted instantly to the magnet, You need to clean the underside of the armerture of SL5. Metho or alcohol is what i would use, but you need to get into it and do it properly and get rid of the sticky junk on the solenoid, no lube required, just clean, Read the manuals I gave you regarding SL5 and the transfer roller.
.I dont think it is a tray problem is is to do with image transfer not paper feed so using the bypass (manual feed) wont help you, but to use it just tell the printer driver in your windows/printers/propertys to use MP tray.
You want to print 1000 pages,!!!
i think it is more economical to use a print shop than fork out for the consumables like belts toners paper drums, once you work out the true running cost of machines compared to high volume machines made for the job, you tend to lose out everytime. In my understanding it better to wear out someone else machine let them take the heart ache and expense of keeping quality supplies up to printing big runs, but thats only my educated understanding, in truth I make more from you buying toners and drums endlessly;) rather than paying 10cent a print or whatever. Work out your running costs, factor in toners, drums, fuser, belts, feed rollers and paper. Get the life of each one and divide it up into per page, you will end up 10 - 20 cents per page. Use original supplies not aftermarket if you dont want to cost yourself more than you save by using crud. Its easy to sell crud aka aftermarket recyle save the planet heaps cheaper printer consuamables but it creates 90% of my service calls @ 120/hour and wrecks machines most days from saving a buck, in the end I take all the savings and customers go off to buy another machine only to do it again when some salesman entices them with cheap promises...
anyway enough of the reality I see but you did ask for my thoughts......, get into that solenoid and clean it up, you should be right.. oh and the forever to spool, try a different printer driver like a postscrip one or something if my guess is right and your forever to spool issue is when you are trying to print Adobe PDF files;)