Strange behavior

I have a 65 with auto, 289 with mild mods, three row rad, supported upper and lower hoses, new hoses and heater core, 16 lb cap on the rad, 6 blade rigid fan with shroud. 14 degrees initial timing and I think 32 degrees over all. Here is the strange problem NO problem with heating up if I stay in the city. NO problem if I stay on the highway. BUT, if I come off of the highway after an hour or so, running 2800 rpm at 70 mph, the heat will go up to just past center on the gage as I dirve in the city or through a slower area stop and go and the tranny will start acting up like it can't decide whether to up or down shift as I come to a stop. The tranny does need replacing. Sorry for the long post.

Thanks, I am trying to hold out untill money is a little easier to come by.

D.Hearne said:

If you've got a C-4 and it sat up for long periods of time, it's got dried up seal damage and the hot oil is bypassing them, making it shift funny.

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This sounds right. It sat for almost two years while I built up funds. Replacing the tranny is already in the plans so I will put that as the next priority. Thanks for the advice! Will also add the suggested cooler for added protection.