In-tank pump bad already?

[2-3 heaven]

So I just bought my 83 242 Turbo, and you guys and this forum have helped me quite a bit. Thank you. Now I am once again stuck. It has been running fantastic, then after work one night, had a hard time starting. It missed like crazy all the way home, 50 miles. It is only on acceleration and puffs black smoke when it does. The main pump has been making noise on and off and I figured the prepump would be due soon anyway, so I wasn't too surprised. However, in going through receipts this morning, I found the previous owner had the prepump replaced in 10/2011. The timing belt broke on him maybe 6 months later. Could this thing be bad already? I had to replace the ground wire to the pump when I first got it as it snapped off at the sender when checking it, so I know the ground is good. I bought a 740 turbo pump thinking I'd upgrade while I was at it. Should I still replace it? I checked the main pump pressure a week ago and it was good. The missing comes and goes, but if I pull the prepump fuse it acts basically the same way.