[chiphead]

Does it do the same thing in reverse when it's cold? If its a fluid problem, it should slip in reverse as well. It could be a sticking valvebody spool valve. Check the fluid, filter and valvebody. Also check for restricted cooler circuit.

What I really suspect is that the tranny is high mileage and the seals have gotten hard in it. This allows fluid to blow by the apply piston and the clutch wont apply until the fluid warms up. Since the complaint is slipping in 1st I suspect the fwd clutch pack.

The forward clutch is the only clutch used for 1st gear. If the tranny is slipping in 1st, that's the fwd clutch that's slipping. It's also used with the intermediate clutch for 2nd gear and the Direct clutch for Drive. If the FWD clutch is slipping, eventually it will fail and you'll lose all forward gears, but reverse will still work. Reverse uses low/reverse clutch and direct clutch.

If you heard any squealing, squalling in 1st or reverse, or found broken sprag parts in the pan, I would suspect a low sprag failure. The low/reverse sprag (different from the low/reverse clutch) is a spring loaded gizmo with rollers used only in low and reverse, to hold the rear planets stationary. But it doesn't affect 2nd or 3rd unless it totally comes apart. If it was the reason, it would slip in 1st and reverse, but the tranny would make noise and put metal in the pan.

The best thing to do is to let the fluid warm up before you try to drive it. If it tries to slip in 2nd or Drive, park the car because the tranny is on the verge of failing the clutch plates and stranding you.

Unless you find something simple, a slipping tranny indicates impending failure.