You're battery is definitely the problem, I've got the same grinder which performs reasonably well for an older style machine when run from old original 2.4 amp nicd dewalt batteries. I bought some what Iooked like original dewalt batteries from eBay and became suspicious when my grinder lacked power and the batteries quickly faded. I then got confirmation that the batteries bought on eBay were fake. The first give away was on the kitchen scales the copy batteries weighed slightly less than the originals. I also then bought a 3 amp hour compatible battery from an ebay seller with good feedback. This battery also was gutless and nowhere near 3 amp hours but by my reckoning probably less than 2 amp hours. I then got an original battery from an authorised dewalt dealer and my grinder then performed as expected, ps I've also used the converters for running older tools with lithium batteries which keeps the older tools working though I'm careful not to over discharge the lithium batteries as the battery protection is built into the lithium tools. Warning! you discharge a lithium battery too much you'll ruin the batteries very very quickly.
