Ulster had to settle for a share of the spoils in a 29-29 draw at Ravenhill, which means Mark Anscombe's men have won just one of their last four games.
A hat-trick of tries from flanker Dean Budd, a single score from Robert Barbieri and a penalty and three conversions from Alberto Di Bernardo got Treviso the draw.
Though they avoided defeat, thanks to tries from Andrew Trimble and Stuart Olding and 19 valuable points from Paddy Jackson, Ulster's slump in form has done considerable damage in their push for a home semi-final.
After leading 20-10 early in the second half, the home side let their lead slip - and their cause was hardly helped by having Iain Henderson and Lewis Stevenson sent to the sin-bin in the first and second halves respectively.
The Italian side sealed the draw in the last move of the game, which could have been a second successive Ravenhill win for them had Di Bernardo landed the difficult conversion.
Di Bernardo opened the scoring with a third minute penalty which Paddy Jackson was unable to match with a long range 12th minute attempt.
He made no mistake six minutes later though with a straightforward conversion when Trimble scythed through after Ulster had kicked a penalty to the corner and moved the ball at pace off the back of the lineout.
He then took Ulster's lead to 10-3 four minutes before the half hour which kick-started the home side into dominating the collisions and getting some good go-forward ball, resulting in substitute Tommaso Iannone being shown yellow in the 31st minute for holding the rampaging Trimble back.
Ulster immediately responded with Olding searing through to score which Jackson again converted. Then, after a great Brendan Williams break to within striking distance of Ulster's line, both sides were reduced to 14 when Henderson was binned.
Treviso hit back next and after Doppies La Grange came close to scoring, a lineout maul saw Dean Budd peel off and score at the corner.
Di Bernardo's lovely conversion made it 17-10 to the home side at the turnaround.
Ulster needed to score first when the game restarted and did when Treviso were penalised at a scrum - Jackson landing the three points.
However, the home side again shot themselves in the foot when Stevenson was shown yellow for a late tackle and straight after his 50th minute departure another Treviso maul saw Barbieri score.
Di Bernardo landed the conversion and Ulster now only led 20-17 - and they were unable to stretch this margin when a long-range Jackson penalty fell just short.
He made no mistake two minutes later to give Ulster a six-point lead while still reduced to 14 men.
However, even with Stevenson restored, Ulster still contrived to give away the lead when Ricky Andrew's clearance was charged down by Budd - who grabbed his second try, which Di Bernardo converted to give Treviso a 24-23 lead.
Ulster struck back with Jackson getting a second chance and after sub Marco Fuser moved for his first attempt which was well wide, the home side's outside-half put it between the sticks to put Ulster two points in front - although Jackson was now in the centre while Ruan Pienaar briefly filled in at outside-half before leaving injured.
With four minutes to go, Jackson kicked his fifth penalty but then with pretty much the last move of the game Budd smashed over for his hat-trick - but Di Bernardo could not kick the conversion to win it.