This is just a guess but it is something I need to check on my mach (1994 VF4). I have had experiance on other Haas machines with the spindle encoder belt getting coated with slime by the fan over time and breaking down the rubber and causing the cog belt to start loosing teeth and/or the pulley fills with rubber slime from belt causing the belt to be able to slip and jump time.
I have also experianced about 1/2 a dozen spindle encoder failures and it causes the spindle load meter to show exsessive load, spindle to act sluggish and causes it to alarm.
Both are easy to replace. The belt is cheap, the encoder is not. I have been told they are off the shelf encoders that can be purchased outside of Haas, not sure if the time spent finding it is worth it.
Just my 2 cents