Loss of power under full acceleration.

I bought a 1986 1.3L Suzuki Sierra last year with 53,000kms on it, besides it overheating when I drove it home due to a leaking radiator the car went relatively well.

Since then I replaced the radiator, bushes under the handles of the transmission shifter handle and the transfer case handle (hopefully that makes sense), new fuel filter, new oil filter, new heater core, had the alternator tested and it's working fine, all new oils.

After doing all that I took the car for a drive and when I got into 3rd/4th/5th gear the car began losing power, engine revs and speed started dropping. I was told I should replace my spark plugs, and I did. After that the car went fine in 3rd/4th/5th however except this time I can push the accelerator down about 90% of the way, if I push it the last 10% the car starts to lose power, if I let it off a little bit to within the 90% it starts accelerating again, foot down the last 10% and it starts to lose power.

I have noticed a spark outside the distributor, and also I rechecked my fuel filter and after taking it for a test drive the fuel filter is really dirty again! I pulled off the fuel pump and I think it looks okay, the only thing worth maybe mentioning was there was a slight mix of fuel and engine oil and the rocker arm was quite hard to push in and out.