timing jumps

i just recently built a 4.3 out of a 98 blazer 4x4. the engine was seized when i bought it so i sent it out and had it linebored and bored out. at first when installing the timing chain, i noticed the chain was a little loose, but didnt think much of it. the engine ran great for a while, but jumped time about 3 months later. i  pulled off the cover and set it back again. and it just jumped again two days ago. do you think when they linebored it they did too much, thats why its jumping time? does anybody know if they sell some kind of tensioner for this type of problem, or possibly a smaller chain?

thats all i can think of. i know i cant throw on bigger gears because thatll throw the timing off even more. and oversized bearings would mess with oil clearence.