When I crank hard it seems to me like the Freehub is skipping (randomly) and making a popping noise. I checked out all other drivetrain components and they are fine. And just to experiment, I used another wheel I have and the bike rode fine. I'm pretty new to the Freehub/Cassette technology as I have been out of bicycles since the 80's, but I do have a strong knowledge of bicycle repair. I was just wondering if Freehubs do wear out like that.
You may also consider trying to service your free hub first. I have had great success in doing this with Shimano LX/XT hubs. What I would suggest is to remove the FH body from the hub (you will need to use an allen wrench/hex key after prying off a retainer or dust cover). This is of course after you have pulled the axle out. What I would do then is FILL the FH body as best you can with a penetrating lubricant such as PB Blaster (this stuff is great because it will magnetize itself to any ferrous parts so it will ensure that it coats the pawls and ratchet mechanism). If you have access to compressed air, I would then FORCE the lubricant out of the other side and repeat till it comes out clean. I would then add about a toothpaste cap's worth of a high quality synthetic motor oil, gear oil, or something similar. You don't want anything too thick otherwise the pawls will stick on the ratchet and you will be back to square one. Unless you have legs of steel or log a zillion miles, or have shattered the base of the FH body, I will bet dollars to doughnuts that your pawls are gummed up with crap and that the above will cure your problems with engagement. This has worked for me every single time I have done it with no exceptions. I like to think I spin pretty hard and log some long rides but I have never shredded a Shimano LX/XT free hub. Bent axle? Yes. Destroyed cassettes? Yes. Torn pawls? Never yet...
I hope this helps. If you need more specific help, please let me know. If you do have compressed air and a small funnel, fill the FH body with lube and then put the funnel over it to create a seal... attach the air to the 'spout' of the funnel and blow that FH out. There's an uber-high zoot gizmo you can buy to do the same thing and I'm sure it's a worthwhile investment but this will do the same job (just more mess). Of course, time is money and both of them are your's. You can always swap a new FH on to the hub for a faster fix.