Could use some help identifying buggy issue

[Lsguy]

Hello all.

I recently purchased a buggy used that turned out to be a Baja reaction 150 version 2. Buggy sat for a long time in a barn. The guy said he got it for his son, who only rode it for one season as he got his license after that and it had been sitting since then. I did not believe the story, but continued checking it over. Turned over, but wouldn't start when I went to look at it. Price was great, so we cut a deal and I took it home anyway. Pulled the carb when I got home, and the fuel that sat in it was so bad that I couldn't get a needle into the jets. Didn't want to mess with it, so I ordered a cheap carb from Amazon. With new carb installed and some new fuel, the buggy fired right up to my surprise. Lurched forward upon starting and would die when the brake was held. I figured the clutch was locked from sitting to long. I rode it around for a bit to make sure everything functioned, it did to some degree. I decided to look everything over since it had sat for so long. The coil wire was cracked heavily from dry rot, plug sparked but very dim orange. Vacuum lines were shot. Fuel line broke when I moved it. Brakes were stiff and the rear caliper was sticking. Shifter very hard to move. This all seemed normal considering how long it had sat. Pulled the cvt cover to check the clutch, and as I had figured the shoes were locked in place. What struck me though, was that I believe the guy was telling the truth about how much usage the buggy had. The belt looked stock, and had no visible wear. The torque marks on the clutch were in tact. The clutch shoes looked new. Inside of the cvt cover was spotless. Brake pads have very little wear. I haven't opened the engine yet to check the internals, but so far this guys story checks out. So I put together a shopping cart and ordered some new parts for the buggy. New shifter cable, New performance cdi, performance coil pack, iridium spark plug, Uni filter, new clutch and bell, 1500 rpm spring, performance variator, 10G Dr Pulley sliders, bigger carb main jet, Gates powerlink belt.

New upgrades installed, buggy fires right up and runs strong. Most everything is working great, except for the one issue I have which due to my lack of experience on the GY6 is out of my wheelhouse as far as diagnosis. Randomly, the buggy makes a clunk sound accompanied by a lurch. When running it on the trail, it'll happen sporadically. When running at max speed on the road, it will do it consistently every few seconds. I have read online that this buggy has issues with a design flaw in the reverse, leading to worn gears and sometimes not engaging completely. Does this sound like the issue, or if not what else might it be? If this is the issue, which parts specifically would I replace to rectify the issue?

Thanks for any help.