Sorry to repeat questions which are no doubt hidden in the archives.
I find myself constantly sliding forward on my bike saddle. So is this a saddle fit problem, a saddle choice problem, or a bike fit problem?

Could be all. But if your saddle is setup correctly, your stem is too long.

Sliding forward when you're at rest, or when pushing hard (common)?

jw

very similar question asked there recently and answered by Steve Hogg.

I am assuming your saddle is completely level (nose to tail, using a carpenter's level device, bike on a level ground). I think your reach could be too long i.e. distance and drop from saddle to handle bars too long - due to stem or top tube or both being too long. It could also be that your are sitting too far back and naturally need to slide forward under load. Or a bit of both.

sliding forward on the saddle is the one thing that bugs the hell out of me, i've been doing it since i started riding last year, but since i discovered the saddle was a bit far back it's not nearly as bad as it used to be. i have wide sit bones, so i find that to make my saddle comfortable i need to sit further back on it, so i slid the saddle forward so i could sit further back on it still have knee over my pedal spindle. so far, so good...

i also switched to a shorter (10mm) stem, because i thought i was sliding forward because i was reaching to far, but after i changed it i was still sliding forward so that wasn't the reason for it...

I know a guy (hes on this forum) that appears to slide forward because he sits up too high on the bike when pushing hard. Instead of getting aero and bending at the waist he sits up like its a MTN bike.

His stem seems to acually be a little short because while in the drops his elbows hit the knees.

Try sliding the saddle back a little and tilting the saddle a little nose up. Sounds counterintuitive but that helps keep you back in the saddle.

Same here.

Just be careful with the tender bits.

I'm the same way. I have to tilt my saddle nose down a bit to prevent numbness, so I tend to slide forward, push back, etc. I've come to feel that it's not a problem, it's a good thing. Spreads out the butt wear. Muscle use is also a little different on the different parts of the saddle.