Safety wise, important to first make sure your machine is grounded and it's not an electrical problem there....but if it's only happening with those two belts -- high bet on static too.
As other poster mentioned, belt sanders can basically be like a Van de Graaf generator. The belt itself has insulating material and builds up a charge on the outside surface of the belt. Some belts - depending on the backer material or abrasive - are worse offenders than others. (Guessing the two belts you're having issue with are possibly different than others you use?)
The zap can be avoided a few ways. The first and simplest is grounding yourself to the machine. In other words, make sure you are touching the metal of the machine at the same time you are working. Any charge you "catch" will then flow through you and back to the machine....grounding out the charge.
If you're working slack belt or can't touch (ground) to the metal of the machine -- then you have to have some other outlet for the charge to escape from you. (The electrons from the charge flow from the machine to you, and if you don't have a place for them to go --zap). Working barefoot -- no heavy rubber sole - on one foot, can solve that. Charge flows through you and back to ground through foot. Some will solve the problem by connecting themselves with a copper wire (touching skin) to a ground....like a leash or tether. there are also bracelets and products on the market that will solve this problem with either resisters or other methods.
A last approach that also works is any of a number of devices you attach to the machine that act basically as "Scrubbers"...they catch the building static charge off the belt and ground it out so that the build up of electrons is never high enough for you to get zapped. (I've heard of people creatign a hillbilly version of this by putting a magnet inside their machine near the belt. Magnet catches metal shavings until they eventually touch/skim the belt....that then helps catch and dissipate static as it builds by redirecting it)
added point -- usually heavy soled shoes don't help and do the opposite by insulating you but you mentioned your boots help. Some work boots are designed to ground small amount of static charge. Often they have ESD in the model name for electro static discharge....but may not. It's a pretty common feature, I believe. Not exactly sure how they do it but believe they're engineered for a narrow resistance range. eg/ they dissipate small static as a ground but not more powerful charges.
did a 4 hour grinding session without any problems, felt really good but got me thinking if it could have been temperature/weather related?
this pre winter has been very various weatherconditions, freezing, plus degrees, damp and foggy and because the grinder is out in non heated garage
well... so far so good and happy it seems to be working ok!
Like was said before I am pretty sure that you were experiencing static charge. I get this on occasion sometimes very bad. next time it happens get a static cuff or static booties from an electronics store if you use the cuff make sure and route the wire so there is no possibility of it getting caught in the wheel/belt. you shock will go away.
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