TH350 Seems Unhappy

[SuperDuperDoughnuts]

Got a '65 Suburban that a previous owner stuffed a SBC and TH350 into; it's been running great the ~2 months I've had it. Took it to work today and it was fine like always. Went out at lunch to grab some grub (first time it's been close to ~80F out and I've driven it) and as it got close to operating temp it wasn't happy. General idle at stops seemed really rough, along with coasting/decelerating. On the gas (light and hard) and seems like normal. I discovered that shifting into neutral or park solve the problem. But now I'm worried about my trip home.

Fluid levels seem fine. Coolant gauge reads 195~200, which is normal. Don't have a tach but revs seem normal when I have some (light/mashed, doesn't matter) throttle input and I'm moving. Just decelerating is a rough idle, and once stopped the transmission seems like it wants to grab. Like I said, shift into P or N and everything is dandy.

Anything I can check before I head home?