'89 Caravan missing 4th gear

'89 Grand Caravan, 3.0, 171,185 miles, already on 3rd transmission.

The van is a wonder, and in fantastic shape, body's great, motor spins like a top.

Trans lost 4th gear, and sometimes doesn't want to go into 3rd.

If driven CAREFULLY and GENTLY, I get 3rd gear... otherwise no.

Last trans from a junkyard (I got stuck on a Saturday nite and had no choice, 1000 miles from home)

It failed after about 4000 miles.

GREAT local repair shop said I needed a new battery (not enough voltage to one of the sensors) and a new sensor... $200 and Monday morning, I got the van back.

Fast foreward 2 years and only 10,000 miles later, and the van is still in great shape, motor is a CHAMP, and the trans won't shift into 4th...

Worse still, with a heavy foot, 3rd gear is gone...

When I get up to about 35 MPH, the engine will surge 3 times (kicking DOWN into 2nd, because of the trans), and then stay in 3rd.

Top speed is about 45, unless I can find a hill (down, that is!).

I am not a mechanic, but I am a "parts-changer"... If I know how to fix it, I can fix it.

I've read about "sensor packs", ATF+3 or +4, and band adjustments...

I need a good clue so I can keep my beloved Grand Caravan on the road.

Any help would be GREATLY appreciated...

TIA!