The 400 Yamaha motor....... Too small and air-cooled.
That's easily solved by changing to a larger water cooled motor.
The design is fine. The construction quality is the problem. The rear swing arm is flimsy and flexes under side loads. There are no bearings in the swing arm. It's just a pipe inside a pipe.
My Pulse had 1/2" of side play before I shimmed it out. The whole front end is flexible. The outriggers move too much.
Try this with your Pulse. I have.
Start driving in a large circle. Speed up and make the circle tighter. As the speed goes up and the circle tightens, the Pulse will do a strange dance. The outside outrigger will start to bounce sideways while the rear wheel leans over due to swing arm flex. The rear tire will lose traction and then spring more upright and then flex over again while the outrigger bounces. This will keep repeating itself at about 4 cycles per second until the whole Pulse is shaking quite violently. It might go right to destruction. I don't know.
If the frame, outriggers front and rear suspension were more rigid with proper bearings this might not happen. The strange Pulse dance is caused by all the flex and slop in the Pulse itself. This I believe is also the cause of the unstable feel of a Pulse at speed. It wanders within the looseness of the front and rear suspension and outriggers. Stiffen up everything and it will improve.