the first thing that all of you should start with is to replace each and every rubber vacuum hose you see in the engine compartment. You can't always tell their condition by looking at them, and they are cheap, so just replace all of them and make sure you have an absolutely airtight seal at every connection. I would use silicone vacuum hose if you can find it, since it seems to shrink down onto whatevery it is connected to and you pretty much have to cut it off if you want to remove it. I cleared up all kinds of shifting problems just by replacing vacuum hose. There is also a chance that the vacuum modulator on the valve cover housing is leaking or worn out - another cheap item - just replace it. You should also clean the ALDA pressure junction (a small threaded elbow) at the back of the intake manifold. And lastly, try disconnecting the EGR valve and plugging off the vacuum line that runs to it, the EGR can also be a source of vacuum problems. Good luck.

Greg