I have a 98 Z28 with 4L60e. It one day just lost third and fourth gear going down the road. Then it was fine awhile. Then it would lose 3rd and 4th gear. I took down the pan, it is as clean as new. The fluid is as clean as new. I put new fluid in anyways. I've gotten codes p1810 and p1870 before. I took a look at the tfp switch. It had a little bit of gunk on it. I replaced it. Drove probably 150 miles, lost 3/4 gears for a mile or two, then had all the gears back. I'm going to guess the pcm has a problem or there is a wiring issue between the pcm and the tfp.

Anyone have any ideas

 

P1870 is a transmission component slipping.
It's probably the 3-4 clutches going out. When it detects slipping it might max out line pressure which will make it drive again. Thats one scenario. It's common, it's wear. It's not common for the wiring to just get messed up. I wouldn't start by tearing the wiring apart.
How many miles on it?
P1810 could be the harness inside the transmission. I've also seen that code come up starting a vehicle in gear (when the neutral safety switch is bypassed).

 

The transmission was rebuilt with the kevlar power pack clutches and steels 30k miles ago. The car will run fine for days of driving and then sometimes loses 3/4 gear. Twice it's been where it would only have first gear, then was shut off awhile and had all four gears for the 25 or 30 miles to anywhere I go in town. For example, today I drove the post office. About six miles. Ran with all four gears and overdrive on the way there. Was inside ten minutes. Come out, start the car, and it has drive and reverse only for the six miles home. Showing only p1810. I replaced whatever that port solenoid thing is called. Seems to be some sort of electrical and wiring gremlin. I'm sure later on, I'll start it and it will run just fine for awhile. Just like the last time it was stuck in first and then worked fine for awhile after it was sitting. Doesn't seem to be any rhyme, reason, or pattern to it.

 

Sounds like its going into drive home mode, 2nd and rev. Could be several things
causing it, power failure, grounds, pressure switch, ECU, etc. If the ECU cant determine
what gear you want, it will go into drive home mode. Can you move the shifter thru the
gears when it fails, get the gears back.

 

Check the pressure. I had a similar issue. Long and short was either a bad epc force motor and or a leaking boost valve. Fix it before you destroy the 3-4 clutch pack if you haven't already.

 

What fixed your issue? I've got a 5.3/4l60 in a s-10 doing same thing. Trans lost 3-4 gear. Had a shop replace clutches and it lost all gears except 1st and reverse after trans warmed up. They couldn't fix their screw up. Took it to another shop and they found a bad pump and a cracked drum. Now it shifts through all the gears but after about 10-15 miles it will shift from 4th to neutral. Drop back to 2nd manually and shift back up and it has the gears again.