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Anyway, Ive been building performance Ford C4's and C6's for years with not too many problems, so I decided to try and build the tranny in my 90 Mazda truck. The technical name for the trans is a 4N71B.

There were no problems, or surprise's with rebuilding the trans. In fact it looks really like a C4 internally.

The trans isn't computer controlled, but it does have two electric shift solenoids. One is downshift, the other is for manually turning off the OD, by pressing a button on the shifter.

After the rebuild it ran and shifted fine just driving it around the block.

The first time I went on a longer trip, was about a 20 miles round trip to the store.

Going to the store all was fine on the highway with driving speeds 55 to 70mph.

On the way back when I got up to 50mph the tranny would start going up and down about 100rpms. The faster I got the faster it went between up, and down. Always just a 100rpms up, then down, etc.

I drove it again another 20 mile round trip. Same thing, on the way there it drove fine, after sitting for about 30 minutes then going back home it did the same thing, just a 100 rpms up and down the whole trip back home. Always in overdrive, but never in 3rd.

My thinking is the trans fluid is too thin I can't think of anything else. I guess it heat soaks when I stop which thins out the fluid. I use Dextron III in this tranny. What do you guys think?  

Sounds like you have a leak in the overdrive or lock up circuit. I would put a gauge on OD and drive and see what it does. Also put gauge on line pressure at idle you should have around 65 psi at stall you should have 150 psi. Your line and od band when you are driving in 4th gear. If not you have leak. good luck  

Good info. I drained a quart of trans fluid out, and added a quart of non detergent 30wt oil.

The trans still did the same, but it was a longer time between the rpm drops.

What could be leaking in the OD? I cant think of anything, but the solinoid, and I've already checked that as well as the accumulator.

I will try the line test to confirm a leak however. thanks, Mike.  

Well today I drained then added one more quart of 30wt non-detergent oil, and the problem has disappeared.

I know there is still a problem some where with the tranny, but for now I'll probably drive as is.

Also, I thought I had a gauge to test the line pressure, but apparently not.

I still can't figure why it didn't have the problem with 20-30 miles then after sitting it would start acting up. Thicker fluid would equal more line pressure, correct?

What I think is that when I first start out the fluid is cool, and thicker. As I drive the fluid is cooled by incoming air, and when I stop for a period of time the trans has time to heat soak, thinning the fluid.  

Well Ive been rebuilding trans for 15 plus years and never have heard of use motor oil in there trans.  Glad it worked for you, hope it don't tear up the seal. There has to be leak at the worn out valve or a seal cut. good luck