Those second-hand electronics shops that sell cameras, PCs and phones etc, can be a good source of monitors. I got a decent DELL monitor to replace the CRT on my old Windows PC for £15.
I would also check the voltage rails: 5V, 12V etc, of the computer power supply while they are under normal load. Some computer PSUs do not have much spare capacity, so the addition of peripherals could be dragging a supply rail down.
The following thoughts might already have been checked or mentioned :
Have you defragmented the hard drive? Might not be an issue with a SSD?
Is the computer main memory 75% or more full? Might be causing processing problems.
With no separate video card, the computer RAM has to process everything and might be getting full, leaving not quite enough capacity to process the video, leading to hiccups and freezes?