GT40 Problem

I have a 97 Supra Sunsport with a GT40. Yesterday I took the boat out and while we were all wakeboarding I noticed that the boat was a little sluggish getting out of the hole but it would still get up to riding speed with the ballast full. After 3 of us wakeboarded we decided to surf and added a little extra weight (probably not more than 1500lbs total ballast plus 3 people) but it would not go higher than 8.5 mph. I thought maybe it just wouldnt plane because of how the weight was setup but then we drained all of the ballast and it still would go faster than 10 mph. After that we idled back to the dock and on the way I started noticing that it was backfiring. It did die a few times but it would always restart and we got back to the dock ok. Engine is a fuel injected GT40. Does anybody have any ideas? It seems like I remember someone on here having a similar issue not too long ago but I couldnt find that thread. Thanks.