i just had the same problem. Turned out to be the vacuum hose. It cracked right at the plastic support on the firewall. I noticed some of my other cables were getting brittle, so i replaced all the vacuum limes. Takes about 3 meters of hose to replace the smallest diameter hoses. Ran me about $10/meter at the dealership. Just be careful when removing the hose form the vacuum motor for the Anti-shudder valve. I broke the nipple off and i couldn't find anyone who sells a new vacuum motor, everyone wanted to sell me the EGR assembly at about $200. I epoxied a new nipple on and i am hoping it will hold.