Did any warm blooded animals evolve back into cold blooded?
Secondary ectothermy has been suggested in living crocodylians by Seymour et al. (2004). Their rationale is based on:


Presence of a 4 chambered heart (otherwise only found in endotherms)
Post-cardiac shunting via the foramen of Panizza and "cog-tooth" valve, which develop secondarily in crocodylians
Lung structure and ventilation during locomotion


Thus ectothermy in extant crocodylians is associated with being aquatic or semiaquatic. These authors suggest that endothermy may have been ancestral for dinosaurs as well.

Seymour RS, Bennett-Stamper CL, Johnston SD, Carrier DR, Grigg GC. 2004. Evidence for endothermic ancestors of crocodiles at the stem of archosaur evolution. Physiol Biochem Zool 77:1051–1067.