300E Stalling Intermittent
I also have an '88 300E which stalled on deceleration near idle. I tried the usual suspects ... OVP, fuel pump relay, etc. However, mine turned out to be the hall-effect sensor located on the back of the speedometer head. My 300E hasn't stalled since I changed that little two pronged devil, which senses the vehicle speed and also provides input to the cruise control system.
If your cruise control is erratic, or doesn't work at all, I'm willing to bet this is your problem. You can pull the instrument cluster and remove this sensor from the back of the speedo head. If the solder at the base of either one of the two prongs is broken and/or the prong is loose, then this needs to be replaced. These were common problems on the '88/earlier 300Es and there is an updated hall-effect sensor now to replace the older defects.
Let me know if you want more information on removal/checking this bugger.