Have you considered using chloramphenicol to arrest the culture (protein synthesis) and amplify the DNA. You can find many protocols by searching the web. You may need to adjust the timing and the concentration of chloramphenical. It works great in my hands.

Actually, you can use any available for you inhibitor of protein synthesis. Chloramphenicol suggested by Zhu is one of them. You may be interested in Spectinomycin http://www.sigmaaldrich.com/catalog/product/sigma/s0692?lang=en®ion=TR - Sigma promises to increase copy number 100 fold after 10-15 hours of incubation with this antibiotic. However, inserts of your metagenomic library may contain toxic for E.coli proteins (most often they are transcription factors and DNA repair proteins), and after increasing copy number of cosmid their toxic effect will increase too (almost any kind of gene is expressed in bacteria). You can do purification of your labrary at two stages: one after short incubation (no more than 2 hours) and one after long period of incubation (10-15 hours).

That sounds great, I'll give it a try! Thank you very much.

Just an update (for those people having the same problem):

The solution to this problem is: use a kit (Qiagene large construct kit).