You don't say what engine you have. I would suggest getting an OBD scanner, you can either buy one or borrow one. Several auto parts stores will let you borrow them around where I live. You have to give them some form of assurance that you'll return the scanner. If there is an intermittent problem it is likely to show up in the log. Or, you can take it to a dealer or an honest mechanic you know and have them look. Intermittent problems are the worst, since you may find yourself waiting for an actual breakdown to know what is really wrong. Some things you can check are whether the engine slows when it loses power - watching the tachometer, do the RPMS drop? Or, contrarily does the engine increase in revs, even though you are losing power. This would be essential in deciding which system to trouble shoot first. If the engine slows, then you might be looking at a fuel pump, or carburation or intake issue. If it speeds up, then a more likely problem is the clutch, though usually that is more consistent than intermittent.