Recently I noticed that my rpm is at different point at certain speed. Usually at 70 mph, my rpm read ~3500, and usually vtec kick in after 5000, but sometime when I'm running at 70mph, my rpm is would read ~3250, and the vtec kick in ~4500. When the vtec kick in ~4500, the car jerk a bit, but I don't feel the power gain, or at least not like before. No cel light or anything. Anyone know whats going on?

95 Accord with h22a...beside the sri, everything else is stock.