My 98 Honda Accord does not downshift to first gear completely if I stop the car and continue to drive again almost right away. It would cause my car to jerk when I accelerate again. But if I stop the car for at least 15 seconds and then accelerate, then there is no problem. Is my transmission dying? If not, what is the cause? Thanks.

I would drain and refill the transmission ASAP. In the future, only use full-synthetic transmission fluid; e.g. Mobil 1 Multi-Vehicle Synthetic ATF. Transmissions work and last much longer. Synthetics vastly improve upshift and downshifts. Easy job on Hondas for the do-it-yourselfer.