One speed transmission

I rolled my 1984 380SE out of the garage this spring (during the winter I just back it out and let it idle to warm the oil and charge the battery) and, while it works fine in reverse, in drive it stumbles badly
until it warms up a few minutes, then happily stays in first gear. I've had it up to 4K and it wont shift.
Is this something the mechanically untalented can look into, or should I just take it to the mechanic?
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