I am not a bicycle mechanic, so I defer to those with diagnostic suggestions for any matters of analysis....but, for what it's worth, Deore 7 speed thumb shifters are pretty common on Ebay and probably not hard to find in a number of bike shop's spare parts bins. If you really would like thumb shifters, I would go to the trouble to find some because they are quite hearty and should last forever. I bought a set (along with a perfect 7 speed Deore LX rear rerailleur) for a 7 speed bike that previously had SRAM grip shifters. Having learned to ride & shift before the days of indexing, I like the tactile feel of the thumb shifters.