i would try using 0.1 to 0.5 uM concentration as I think that you are using too much primer. Start wth a Ta 5c below the Tm but make sure that you cacculate the Ta and Tm only on the bases that anneal to your template dna in the first cycle of pcr, Any bases added to the primer to give enzyme cut sites for cloning should be ignored in this calculation. If that fails then try using less primer but also adding dmso in a range 0 to 10% if the primers are high GC and likely to have secondary structure problems