Very strange problem with the Trans. NEED HELP PLEASE!

I have a 1993 Mercury Tracer, (station wagon) 4 cylinder (2.3 liter) and the car has 146,000 miles on it. The car has a "new" tranny taken from a junk car, the original had a major meltdown and it needed to be replaced, so my mechanic friend replaced the original with one from another Mercury Tracer which had approximately 70,000 miles on it. My problem is, the transmission (they call it a trans axle I guess) is acting up. It takes off in first gear fine but won't shift. If I put it in neutral then back to drive (the drive that shows the "D" in a circle) then it shifts to the next gear. BUT, after a few seconds it slips out of gear. Then I repeat the process and again, it shifts to the next gear only to slip out again! Now, after awhile it will down shift by itself, stay that way for a few seconds, then it will straighten out and shift through the gears just fine. Sometimes it takes only once or twice to get it to shift correctly, but other times it will do this for up to 5 or 10 miles. It is strange and frustrating, even to my mechanic who is very good. The gears do not slip, it just pops out of gear. It's very annoying and also I'm afraid it might be a prelude to a major problem. Do you have any suggestions? Thank you in advance! (P.S. I am a disabled person on a fixed and very tight income, but will make whatever donation I can when I receive my monthly check for November 2007. Right now though I desperately need my car).