I would be more worried about the timing of the engine sprockets marks being on their targets!

How long since the tension was adjusted.

How long since the belt was replaced.

They do age and stretch you know!
They should be readjusted about every other oil change, synthetic oil or not!

If I run my oil out to the recommendations of 7,500 miles, for Dino oil, I pop the front plug and loosen the bolt every time. A minute or two well spent.

It sounds to me if the belt has not jumped a tooth on one sprocket already it's about to!

I would check the distributor's rotor location before and after resetting the timing belt tension. Do this to just see if moves to set the rotor any closer to being under a wire terminal of the cap.
The cam mark can be peeked at too by loosening the upper half and peeling it back a hit.

From there:
Backfiring is a timing issue or a plug is getting too wet with fuel.

Bad ignition and timing components are first! Then think a leaky fuel component for some real hard starting problems.

Good luck and post back any findings!

Phil