shuddering Tercel on the highway...

[1986Tercel]

Hello all... I have an 86 Toyota Tercel. Had it since '92 and have been very happy with it.
Just recently, she's been acting strange in that when I'm on the highway (on a slight incline about 5%) travelling at about 85-95 km/h (50-60mph), the engine starts to vibrate badly (not the steering) but the engine and there is a loss in acceleration. The funny thing is if I press the gas pedal, the transmission gears down to second and the shuddering goes away but as soon as it shifts back to drive the problem comes back.
Each time this issue shows up on the same patch of highway (with the 5% incline) and always on the way home. On the way to work (in the morning) the care does not exhibit this problem.

Very odd huh... anyone seen this one before?

Thanks in advance...

Regards,

Carson