The monitoring parameters do not mention heart problems:
</font><blockquote><div id="quote"><font class="small">Quote:</font>
Body weight; mental status for depression, suicidal ideation (especially at the beginning of therapy or when doses are increased or decreased), anxiety, social functioning, mania, panic attacks[/i]
http://www.umm.edu/altmed/drugs/bupropion-017700.htm

I would trust your doctor or question him more closely if you are concerned? It sounds like your cardiovascular system is fine (or you'd notice symptoms other than just increased heart rate) and well trained and I don't know that I would necessarily attribute the better times to the meds; could be backing off because of your depression helped; often that is the way with weight loss, etc. Our bodies need rests/breaks when confronted with plateaus and manage to take them and then go to the next level. I think you're just at the next level for you.