Hi, all!
No, I don´t mind people chiming in on this one, often that even gives additional info, and I certainly don´t have a monopoly on, or all, the answers.
Panacur, or fenbendazol, is one of the safest products I know of, and very rarely causes any side-effects on its own provided you´re in the region of the approved dose (usually given as 50-75 mg/kg cat for two or three days, but this may vary with where you are and parasitic load: your vet knows this). Cats tolerate much higher doses.
I am not sure why you are getting diarrhoea with blood in it when using Panacur, but one reason may be the effect it has on the parasites, killing them and thus upsetting the balance that is supposed to be in the intestine. You mentioned cow-pat-like faeces and earlier giardia and coccidia: maybe a revisit to those is in place, you can easily get reinfections with them. I´d definitely go for a new check-up, and get some faecal sampling done for your vet to inspect. If it´s all gooey and liquid, get a syringe to draw it up with, and preferably not from the litter-box if its got clumping litter in it!
Apart from that, probiotics with lactobacillus in it never hurts (usually does good), and TLC always helps. Also, remember to make sure the furry thing gets enough water/fluid, and sometimes a short break in feeding helps (the intestinal wall/mucosa needs about 36 hours rest to reform/regrow/mend) followed by probiotics and incremental doses of food until normal amounts are given. But you really need to retest for parasites!
Good luck, and keep us all up to date!
Per