That's actually a good thing, it eliminates the RF grounding of the antenna because if that had been the issue you'd have noticed the SWR changing when you changed the length of the coax. I can guarantee you it will not be the antenna itself. They'll easily handle anything reasonable you can throw at them and you'd melt the RG58 before you killed the antenna. As loosecannon has implied you need to use an external SWR meter between the amp and antenna to check the SWR is still Ok there. If it is then the culprit is the input matching circuit tuning on the amp.