what i have is a 00 silverado 5.3l, i did a heads and cam package from elliots portworks, so im kinda new to the ls family, dont know to much about them, i think the cam is like 214/218..582/585 with 112lsa..something close to that if that isnt the correct numbers, also had my factory heads ported, up graded to a stock ls1 valve, when i put the truck back together i used felpro gaskets from oriellys..i didnt know anything about pushrod length, or how to check it..so i put it all back together with the factory pushrods..kinda dumb thing to do i know...after driving it awhile i got a buddy to help me check the pushrod length,after the truck got where it wouldnt start like it should, and it never has run like i thought it should..i would have to spin the engine over for 5-10 seconds before the truck would start..so i kinda figured the pushrod length was to long, so we checked it, after bring the pushrod to 0 valve lash,i i could get 2 1/2 turns outta the rocker arm bolt before it would torque down..used a cheap pushrod length checker showed to be like .80" to long..so i shimed the peadastols with a .60" steel washers..truck ran and preformed alot better..after a few weeks of driving it, now its throwing a random mis fire code and still hard to start.. so i pulled the valve cover off and had a buddy spin the motor over while i watched the valve trane, when he spun the engine over the rockarms moved up and down like they are supposed to do, but as soon as he let of the key and quit turning the engine over, all the valves immediantley closed, the lifters are bleeding off enough that when u look at all the rockarms they are all in the same position, like there not even tight, also has alot of valve trane noise..
if someone could give me some advice on whats wrong and what i need to do to fix the problem i would greatly appriecate it.. also do you guys think that would hurt my preformance quite a bit?.. truck runs 8.42 in the 1/8 mile on motor now..

 

You need to take off the rockers, lose those shims, set up the valve train geometry (wipe and preload) from scratch, and be prepared to actually buy some pushrods of the proper length. If you are not able or comfortable doing all that yourself find a good shop.