Not all coax is the same and it comes in many different tolerances to signal loss, however the weakest point of all coax is probably water intrusion. This could be why your fix worked for a while but is now failing. If the existing coax has been exposed to water, any sort of splice/repair might just be a band-aid and the water damage will continue down the line. If your line is rated for direct burial you'll probably be fine, but some coax insulation (the white part around the copper) is open cell foam that can wick water very rapidly. In this case you'd be best replacing the entire run, especially if the signal is being used for high bandwidth applications like internet.
On this note, not all electrical tape is created equal either, so be sure to get a roll of something quality before you do the repair.