A longshot, but I'll mention it. Suppose a PO had the trans replaced with one from a gasser. I forget the mechanism for controlling this, it's somewhere in the valve body. Might explain why it usually starts in second. Perhaps something in the valve body is hanging/changing that is beginning to have it start in first. Perhaps if you got the # on the valve body someone could identify it.

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.