Ever since I have lived in my house the master bath toilet has never worked very well, (easily clogs, slow flow/flush action, etc) After several years of this I went into basement to check out pipes and notice that the initial downpipe from toilet flow to a pipe that slopes well for about 5 feet then turns 90 degrees and then has a upslope for about 2-3 feet. (Checked with 3' level) Could the upslope be the cause of the disfunctional toilet? If so, should I have a plumber re-install the line?

Quite possible with an upslope. I would pull the toilet and pour water down through the floor flange. If the water backs up you have your answer. It could be trapped debris too.
On any event, the pipe should always slope down. What is the pipe material?

Material is PVC

Well, the good thing is that it is easy to work with. If you have decent DIY skills this is not a daunting task. 1/8" drop per foot.

That "upslope" In your drain line gave you a built in running trap. This would cause back pressure and give you the problems that you now have. Follow Brians advice and have it redone with proper slope. Good luck, Tomk

Well, the good thing is that it is easy to work with. If you have decent DIY skills this is not a daunting task. 1/8" drop per foot.

Thank you very much

That "upslope" In your drain line gave you a built in running trap. This would cause back pressure and give you the problems that you now have. Follow Brians advice and have it redone with proper slope. Good luck, Tomk

Thank you very much