I currently also have a love/hate relationship with my cyklons. I kinda don't trust the first stages. They tend to get very creepy after a while.
I've had it several times if I haven't been diving for a while and I test them on my tanks, that the second stage also starts stuttering like a scooter bike. The only way to fix this is by getting them soaking wet first. (I feel like when I dive regularly they function perfectly).
I would occasionally run into similar problems with older Cyklons, that were in need of service.
Almost exclusively, there was some particulate matter within the first stage, that had managed to damage the Delrin HP seat, either from a fouled air source, or from clumsy unhygienic repair. Scratches to sealing surfaces will also cause that issue. A worn piston is another all-too common problem; and I usually replace them as a matter of course, since they are relatively cheap; can otherwise quickly trash a new seat, and cause that IP creep. If in doubt, I'll usually examine those bits under a stereo microscope. More likely than not, I'll see obvious wear.
This was a problem, well known to techs for decades (Harlow even discussed it in his 1999
Scuba Maintenance and Repair, if I recall), but strangely hadn't found its way into any of the collective Poseidon manuals, whose girth already rivaled old Manhattan phone directories.
Establishing a stable IP to factory specs, will also solve some of the issues with the second stage. Mine certainly do not "stutter." There can also be some detritus under the inhalation diaphragm, not allowing for a complete seal . . .
I was thinking of running them with xstream first stages, but haven't yet because of the price
We've also attached the Cyklon second stages to Xstream firsts, years ago; and they do function quite well. The combination is even approved of, in the service manual -- just crank that IP; and Poseidon is now marketing that very combination as the Cyklon X -- a couple of decades too late, in terms of marketing, in my opinion . . .