Steve is right.
There's no reason you should change the carb cover. These run great without that. Some dynamic from active flight that's not present on the ground is the missing link in this mystery, and you need to unravel this mystery to solve it. The motion has to be messing with the gas flow, or the gas setup is flawed. In situations like this, I've seen vent lines hooked to the carb, and everything was fine until the plane started to move around. And THIS one I did myself: I set up the tank perfectly, with the vent up, and slipped it into place and secured it. It looked perfect. But - uhh, brain cramp, the plane was inverted when I put the gas tank in. So, it didn't exactly work the way I intended when it was rightside up again, and the vent was suddenly at the bottom. In another one, the tubing from the cap to the clunk was loose, and once the fuel level started to drop, the clunk line flopped around and allowed air to enter, and it stalled the plane. It wouldn't do it on the ground, because the pickup tubing wasn't flopping around.
The one thing about gassers is that the gas system has to be PERFECT, or it'll act up. Do you have a filter inline? Remove it, and test it without the filter. I've had filters with air leaks that caused the plane to stall in the air after running perfectly on the ground. Every single tubing connection is another place where you can have an air leak. If you don't have collets to slip over the barbed connection, use good zip-ties like 3M, and make sure you zip them up TIGHT around the barb. SOMETHING is happening when that plane starts to move, or climb, or......????? Lines that flop around are moving, and moving lines can lean one way and cause an air leak at a connection. Do any of your connections have just one barb? Those are likely to bleed air. Zip-tie them good and tight, behind the barb. Are you using a gas filter that comes apart for cleaning? Take it out of line, those filters often bleed air where they separate. In fact, take anything out of line that doesn't absolutely have to be there, like fillers, valves, tees, dots, etc. If the problem stops, start re-installing the removed items back into the system, one-by-one, until the problem starts again. Once it starts acting up again, whatever part you just put back into the system is the culprit.
If you need collets, find a local dental equipment supply company, and go buy some 1/4" collets from their service department. If you don't have anybody like that near you, you can find them online. If you live in or near San Antonio, I have them.
I'm betting it's the fuel. Even so, replacing the spark plug again is a cheap bit of insurance.
Before you start changing the carb, troubleshoot the hell out of your fuel system.
Jim