Yesterday I had no repeat of the problem at all, during my 160m daily commute... I hate this intermittent malfunctions!
Thanks for your suggestions, Suginami. I will try to look into it this week end and post my findings later.
85 C300CDT, 128k miles, daily driver
94 C280, 200k miles, daughter's
90 BMW 325iS, 218k miles, FOR SALE
99 Suburban LT, 99k miles, wife's and 5 kids
78 F150 XLT Lariat, 35k miles, farm truck