Moment of truth.....been able to resolve the issue today. Feel kinda ashamed actually....it turned out to be a wrong spark plug, just couldn't figure out. Surprised it was giving a feel as if something was getting stuck on high speed. In the bargain following things or issues got highlighted and sorted. I carried out wheel alignment with thread and found that front wheel is off, handle wasn't straight, rectified. The rear snail cams aren't equal...equal number of notches were making the wheel go right...rectified. Chain was cleaned and well lubricated, the sprockets and chain have lot of miles left. Happy its running beautiful again!! Thanks everyone from valuable suggestions and support!!!