cheers for the reply but yeah its not overheating as it can be a few minutes into the first print of the day as well as long prints.

Possible a bad wire or loose connection. it does not take much.

Second of all I understand most of us are not made of money. However the cheapest choice is most often not the best choice for anyone. The import market is flooded with cheap printer kits and some printers that are just not worth brining home they are nothing more than a collection of parts that when assembled resemble a 3d printer they will not print reliably. No R&D no engineering no testing.

if cooling is not helping, and you swapped the problem motor to rule out that specifically, then pretty much the last possibility is with the board itself.
Does the board have removable stepper drivers?
if it does, then i would suggest swapping out the problem axis with another, and see if the issue follows the driver. If it does, then you can replace the bad stepper driver.
If it does not have removable stepper drivers, then the entire board would have to be replaced.

The correct way to do things is to do your research before buying anything. Look at the best you can afford, not the cheapest.
Find out what other people are having problems with, or not having problems with. Is there customer support for the brand? is there a solid community of users where you can get brand specific help?
Buying cheap, thinking you will learn & gain confidence from it and then later get a better quality machine is the absolute WRONG way of doing this. As you are finding out, cheap often has problems, frequently problems that cause enough frustration to give up entirely.

my other half researched for months before finally deciding what to buy back in 2015, before the glut of super cheap printers hit. He ended up buying a used Solidoodle 4 for around $400, which brought us to this community where I learned everything I possibly could about the machine we had, how to make it work right, and how to fix things when they went wrong.
We still have that printer, and it is still running on a nearly daily basis 5 yrs later. Was it the cheapest route to go at the time? nope, not by a long shot. But it was a well informed choice.

so maybe in time i can be confident enough to get better printers knowing that i can build and sort them.

and that right there also says i want to be confident before buying more expensive so i know im not wasting money especially at this time but since you chose to ignore that and attempt to call me cheap i thought i would spell it out for you again......

i have a fan going on both the motor and the board it makes no difference.
i have also swapped the motor with a different one. and the same thing happens so ruled out a bad motor also.
the motor works fine on the other axis but the swapped in motor jumps

...decided to get a cheaper bigger printer...

and that right there is about 95% of the issue. Stop being "cheap"!
If you want a decent printer that is going to run without too many issues, you need to spend a little bit more and buy something of quality.

my guess is that it has either a bad motor that is failing intermittently, or the board is getting hot and causing the driver to miss steps.
It does not take much to overheat either a board, or a motor. If you are positive the motor is not getting hot, get a fan blowing on the board and see if that makes any difference.

got auto levelling etc working great and when prints do work they are 99% perfect with heated bed / fiberglass belt / PEI sheet and a better hotend than it came with.

the bed started jumping during prints sometimes 5 minutes in or sometimes 5 or 6 hours into it. get a grinding noise and it continues printing on a different spot on the bed.

i read up on it and done a few mods
bed stabilizer mod, belt tensioner mod and axis non wobble mods to name a few but no matter what i do its still doing the same thing.

some prints go fine and print perfect but then there is days where the bed / belt constantly grinds / jumps and the prints ruined

its driving me mad and any advice would be great to get this figured so maybe in time i can be confident enough to get better printers knowing that i can build and sort them.
cheers for now