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.

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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