4runner 1988 power losing

I've 1988 4runner 2.4i SR5 EFI witn 22RE engine. At first I had problem with 'check engine' light at high rpm and speed (but with min power losing), so I changed air-filter-problem gone for a while...
Then I changed fuel-filter, plugs+wires, engine oil, gear oil-acceleration doubles, engine went fluently-but only first 200 km...
Then damn 'check engine' light came up with rapid power down (about 25 km/h down, when I passed by long long truck, so I nearly crash to car ahead...). After while, when I slow down, light went off and engine run normal. But when I tried to accelerate light+pwr down came again and again...
There is no problem during drive without speed gear or down-hill, but for 2WD problems starts at 5. speed gear - 90-100 km/h , 4. sg 80-90 km/h
4WD 5. sg 80- km/h, 4. sg 70 km/h., 3. sg 60 km/h...
and without 'check engine' light engine run rough at 1., 2. sg - low rpm

I checked air-filter, plugs, wires, cables - everything seem to be OK...

Pull error code 51 ???...