I've had similar issues with the Pomona silicone leads (Fluke TL71 style, which are sold with various brandings). Ended up buying another set though, as the price, usability and availability were right. The old-style Fluke leads (TL75) seem more rugged and (I recall) have slightly lower resistance, but in my opinion don't feel as nice in use due to PVC insulation.
Annoyingly some of the other Pomona products are let down by similar issues - e.g. their mini (3A) probes with replaceable tips are really useful for electronics (as opposed to electrical) work, but seem to suffer badly from high contact resistance developing between the replaceable tip and it's holder. Will need to investigate other brands in the future to try and find a more reliable replacement.