1988 Chevyenne driveability issue

[biggsteve97]

We have an 88 Chevyenne 2wd 350 TBI that weve spent the last two months tryng to get up and running. First the engine had no power, found the distributor was off 2 teeth fixed that and got the timing set to 2 degrees. Ran strong and very smooth until a couple weeks ago. If you are accelerating it runs fine, at idle its fine, in lower gear and rpms high its fine. But if you try to cruise and rpms are around 1000-1300 once operating temp is reached, the engine starts cutting out and running extremely rough. Above or below that it runs perfect. Checked timing, good. Checked for vacuum leaks, good. Cold then engine runs fine its just when it reaches operating temp and ONLY at 1000-1300rpms. Changed the TPS Monday night with no effect on the problem. Could this be a failing ignition module? Could by some weird chance since the fuel guage doesnt work, be low on fuel? It would be odd if it was since its fine at other rpms, I would think. Could it be a fuel pressure issue? I am going to work on it as much as possible on Saturday and even hook my laptop up to it to see what the computer is seeing/doing. Engine has unknown mileage, supposedly bottom end is rebuilt. Oil pressure is good. Did notice while engine is cold a little blue smoke coming from right hand exhaust, but goes away when engine is above 130 degrees. Any advice/thoughts is welcomed. Thank you.