I have a 90 SC 5 speed. Transmission rebuilt several years ago and ran fine. Car sat for about 4 years (about 2,000 miles put on it during that time). Recently I started driving more and noticed when shifting that as long as it's a slow easy shift through each gear at low rpm's, there is no problem. If you shift more aggressively, such as accelerating onto a highway, the car will buck / shudder initially in third gear, then accelerates fine. The buck can be harsh (you can hear engine knock) if you're accelerating hard. Any thoughts on what may be the culprit?