When there is a sudden change in shifting like that it has to be some broke or came loose. I would check the bowden cable. It is a cable that is attached to the linkages on the top of the valve cover near the firewall, and goes down to the passenger side of the transmission. It controls the RPM at which the trans will shift. Make sure it's attached on both ends. Then I would check to make sure you didn't loose your shift linkage bushings. When you loose these it can be tough to get the car in to gear properly, which could keep it from shifting properly. If these check out I would try adjusting the bowden cable to see if the issue changes. If it does then the cable it probably the isuse. If nothing changes, then I would start looking into trans fluid and filter.

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