I agree with R P Ryan,

My 190E 2.6 was doing the same thing. I went crazy looking for the problem. One day while looking, my non mechanical son says, "do you think it's the radiator cap?" , I thought no, but changed it anyway....fixed!

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Jeff

97 E320, ours 217,000miles
91 190E 2.6 daily driver 242,000miles
74 MGB,mine 120,000?
2006 BE350/Proline 21, 50 hours