I have a 95 toyota corolla automatic with 177,000 miles. When its cold we have to let it warm up or it will not shift into overdrive. If its hot outside and we have drove it for a while it shudders after a stop and between gears. I have replaced the plugs, plug wires, fuel filter, fuel pump, trans fluid, and the trans filter. I am at a lost as to what else to do. I figure if there was something wrong with the transmission it would act up on a consistent basis... Any help would be greatly appreciated.

Now you may think this is silly, and overly simple. But it may be that you are not topped up with your transmission fluid.

Drive your vehicle around until it's warm (10-15 mins ?)

While it's idling (that's very important, do not shut it off)....now check your tranny fluid. Top it up while it's running to the top of the line.

I have had this situation....and thought that the fluid was enough...afterall it was in the cross-hatched area of the dipstick. But you would be surprised my friend !!!!