I'd find the jumping belt idea questionable unless it wasn't installed right to begin with by whoever last changed your T-belt but then you'ld notice a power loss too. I have heard where cars with manual trannys where the customer parks the car on hills, in gear can cause the belt tensioners to fail but I've never seen it myself.

Get the cyl head off and see if it's just a burned (not bent) valve as I've seen burned valves in 850 engines before. The cause of burned valves is usually either poor quality engine oil, lack of changes, and/or low speed, low rpm driving where the valve lifters don't turn in their bores as they should thus causing them to stick.

If it turns out to be a burned valve, I'd suggest using synthetic oil to prevent future problems. If the valve is bent (and only one would be very unusual) caused by a t-belt that did jump, then by all means replace the belt tensioner too. Any time one replaces a t-belt, one is suppose to check the t-belts tension first to know what condition the tensioner is in. If there's oil leaking from it, replace it always.