PKa:
You are not the first to have these problems....there are many possibilities.
How heavy is your plane? What elevator throw? If you have a heavy enough plane, and enough elevator throw, it will snap out when you pull it....
In a snap roll, the plane is jerked up violently, and can lose some of the rudder/fin effectiveness and yaw stability. Which is why a good snap looks so cool.

If it is only 7.5 lb and your elevator is about 20 degrees or less, it shouldn't snap out of a 20 foot radius loop, IMHO.
My plane is unusually light at 6lb 10 oz and doesn't snap out at ~30 deg of elevator. Instead it loops on about a 15 foot radius.

In that case I would look at what nrad2000 said for sure.... lateral balance is probably the most neglected setup detail when building a plane or modding a plane. Another front runner would be wing incidence: differences of a degree or more, which could cause one wing to stall sporadically, depending on wind, airspeed, etc.
The instructions in my new one (yep finally gave the original one a terminal dirt-bath on Monday!) suggest that you check lateral balance by hanging the bird upside-down using a string running from the tailwheel wire (pivot) to the crankshaft..... I would suggest that you start there and see if it helps. Another check for lateral balance and Front-rear balance simultaneously is to put a screw eye right at the desired F/R CG, right in the L/R center and hang it from that point upside down. Maynardrupp had to add a popsicle stick to his so that there was something to attach the screw-eye to.
If you have a lateral balance problem, flipping inverted is also a good test if you are comfortable flying upside down. A "Left Heavy" plane will have some right aileron trimmed into it to hold up the extra weight. When you flip it over, and try to fly straight, the heavy Left wing is now the right wing, and the compensating right aileron will make it roll off of horizontal repeatedly to the right..... seeming out of trim.

Also, leakage of one side's airfoil surfaces at the aileron or elevator joint.... i.e. RH side tight, LH side has a 3/32" gap..... could cause sporadic response in an up elevator condition. One wing might stall sooner.

I seem to remember Maynardrupp talking about this ca. 2010 on his .75 powered Phoenix Extra- I think keeping a little power over the top helped maintain direction. Start reading here:
http://www.rcuniverse.com/forum/m_47...57/key_/tm.htm
thru at least page 59... notice Da Rock's comment about limiting throws to avoid snapping out. Reducing weight helps as well.....

Some other things to think about.... is this a new type of plane or flying for you? I remember in early hovering practice I used to trim the rudder for "no rudder" hovering... but that rudder setting made the loops kind of wonky!