help w/service engine soon light

i'm new to this site but i need a little help from the mechanics out there! i have a 99 silverado with 5.3L 82,000 miles and the service engine soon light has been coming on recently. i scanned the PCM and the code said mulitple misfires detected. i change plugs, wires and airfilter these things worked for about a day and the light came on again. i did a compression check and found that my left bank is only doing about 75psi at normal operating temp. i was wondering if anyone has any experience on these 5.3L engines with some advice before i pull the head. i read about some intake manifold leaks on this site so i will check that out first. thanks